TL/DR Here's a list of mechanical errors and ways to fix them with practice.
All right, enough's enough. Time to talk about mechanics.
A lot of people seem to think that this is the part of the game that you can't train, like size or running speed in other sports. You either have it or you don't. Well, those other sports are right, you can't make someone taller. But here...they're wrong, and I'll show you why.
Go get a piece of paper and write your name three times as you normally would, but quickly. Do it as fast as possible.
They may not be pretty, but they're probably legible. Now, switch hands and do it again, using the hand you usually don't write with.
Looks like a train wreck, doesn't it?
But hey, it's supposed to. You're either right- or left- handed, maybe even ambidextrous. But, there's the hand you write with. You've been using it your whole life. The other, non-writing hand, gets used almost never. The ratio of writing experience between your writing hand and non-writing hand may be hundreds of thousands to one.
If you'd like to see about developing off-hand writing skills, you can go here, but let me save you the time: you practice. You just keep doing all of your writing with your off hand until it starts looking legible.
Fortunately, League of Legends requires typing (that you're already kind of used to) and right-click mousework, where your hands are probably less used to. I bet you do most of your normal left-clicking with your index finger, while your middle finger gets used less. Furthermore, unless you like to share your feelings with other drivers in traffic, your middle finger just isn't used that often in life.
But hey, that's good news. This means you can train it better.
So, I'm going to try and provide you with some knowledge about mechanics and practice. I'm going to start by outlining mistakes and explaining what they are, and then include a solution for each mistake. Fortunately, many mistakes can be fixed by one solution. I'll try to outline them all.
CS ERRORS (Slivering, Late-hitting, Splitters)
We'll start with what everyone thinks about with “mechanics”, simple CS errors. For the most part, you can click a dying minion and not get the gold in one of three ways. You can sliver the minion, meaning it has a sliver of health left after the auto and another creep takes it. You can do the opposite and late-hit the minion, where your champion's auto attack animation starts. Finally, you can lose a minion to a splitter. What this means is that two minions are dying at the same time, and you can only auto one of them, while the other dies while your auto attack is on cooldown. You'd have to use an ability to take the second.
Solution: Watch the caster and tank minion projectiles.
If you're going to lose a minion in lane, something on your team stole it from you, and most likely, it will be the higher-damage caster minion. That's what you're competing with. Fortunately, the projectiles are visible and relatively slow. You can see them coming. On top of that, you've been shooting minions recently and can see how much damage your shots do, even exactly if you have the combat text on.
So, when in doubt, you look at the caster projectiles. If the remaining HP is high and you might sliver it, wait for one or two more projectiles to land, then shoot it. If you're risking a late hit, get your bullet or punch there before the casters do.
Finally, regarding splitters: the minions don't really change targets away from a minion that often unless someone distracts them. What this means is that a splitter actually has some development time, and if you are watching which one your casters are focusing (or two in the case of a splitter), you can auto attack one of them early. This ruins their HP balance, makes one die after the other, and makes both of them easy to collect.
CLICK TIMING FAULTS (Auto Cancel Faults, Missed Auto Resets), ABILITY RECOGNITION
I've seen a lot of lower ranked games where the players are playing excited, with high pressure. Their actions-per-minute (“APM”s) are higher than they're comfortable with, and they make erratic movements. One is the auto cancel fault, where they intend to auto attack something but move before the animation completes, canceling it. Jinx is notorious for this, as her quick “tzzzt” minigun attack is easy, but her “fuh-woomp” rocket launch takes longer to fire. If you're used to quick shooting with the minigun, you can very easily cancel a rocket shot. This is pretty costly, as her long rocket range means that this auto attack was probably free damage on the enemy, but you lost it because you clicked too fast.
Another issue is if you're playing a champion with an auto attack reset, such as Wukong, Nasus, or Vi. Each of them have abilities that empower their next autoattack while taking it off of cooldown immediately. Naturally, you want to use these abilities immediately after a previous auto attack, so the reset saves you as much time as possible. But, use it first, and you will save no time. Use it too early, and you may even cancel the previous autoattack, which is damage you deserved but didn't get on the enemy.
This last one may be unrelated, but the solution is the same: recognizing when your opponent uses an ability. Playing the cooldowns is paramount if you want to succeed against a competent laner, and making sure you don't miss it is important. Also, most of those bastards are you know...trying to kill you, and will use their abilities when you would be otherwise distracted (taking a CS for example). If your eyes are focused on your CS work, you could very well miss it.
Solution: Turn the volume up
We've already established where your eyes are, either on the caster minions or the minimap. Your eyes are doing a lot of work, and the more pressure you put on them, the more likely they are to miss key information. Instead, use one of your other senses: hearing.
You should be familiar with practicing with your champion in custom games, but head back in there, make sure the sound is audible, and give it a go. Try to push your champion to the limits by canceling autoattacks while listening, and pinpoint exactly the moment when you can cancel and get credit (have scrolling combat text on, so you can be sure) and when you'll get a full cancel. After that...try it with your eyes closed. Attempt to shoot or punch the jungle wight 10 times blind, then open your eyes and see how much damage you actually did. If you did 10x you attack damage, great! You're all set.
To do this, you might want to use the shift key. Since you won't be actually able to see your click, fire at the wight once, move your cursor to open space (away from the wight). Without moving the cursor, shift-right click to shoot, regular right click immediately afterward. Keep shift-clicking again, to return to your AA cycle. Just try to use a move command in each, which has a chance of canceling your auto, but won't if you time it right. Also, you may want to find a LoL inclined friend to help you with this by watching over your shoulder.
As for ability use, this is actually the easiest one. I defy anyone who isn't hard of hearing to miss Sivir ulting with the sound on. But, as you play more, you'll develop the skill to detect ability use and respond accordingly. You will be VERY proud of yourself when you flash that ganking Lee Sin's Q when your attention was completely elsewhere a moment before, just because that “heeeeUH!” was heard and your brain knew what to do.
I play a lot of Ziggs, and I am always quite embarrassed when I airball a Q. What's worse is when it bounces short, OVER the head of the target, and keeps bouncing behind harmlessly. I feel so bad when that happens; it's like I had a wide open receiver in football, and I straight out flubbed the pass.
Solution: Aim for the feet.
The search bar is a wonderful thing, ain’t it?
This game will score skillshots as hits if they intersect with the target's “hitbox”, which is the space they occupy on the game map. This hitbox is not the champion's body, in fact, it's where the champion is standing. Isometric points of view do this. So, with big skillshots like Syndra or Corki, aim at their feet, and you won't miss.
WALK FAULTS, DEAD FOOT FAULTS, SKILLSHOT DODGING, CURSOR LOSS
Here's four mistakes that don't seem all that related, but are fixable with the same solution.
A walk fault is the one misclick that can kill you. It's one of the scariest things that can happen to an ADC: you click on a target to shoot it, but you missed the champion and instead clicked on the ground. Instead of shooting from distance (what you wanted), your champion is now doing no damage and walking TOWARDS danger. Instead of shooting, you're walking forward...boy is this bad. This is how you turn being on offense with momentum into a complete failure.
The opposite is much less punishing. It won't singlehandedly ruin games for you, but you'll miss a lot of opportunites. It's when you've moved in, kiting forward, autoattacking as you go. You click the target to shoot, but instead of clicking the ground, you click the target again. As your champion is already in range, he or she will stand still and sit idly as the AA timer resets. This is a dead foot fault, when you want to be walking forward during the AA cooldown, but you aren't because you misclicked the champion instead of the ground.
On the other hand, skillshot dodging is paramount. You're on defense in this situation and you need to make sure that your opponent's shot will miss. There are very many skillshots that if landed once, will win lanes (Blitz hooks or Nami bubbles) or even entire games (Sejuani ults, Amumu Qs). Naturally, you need the presence of mind to drop what you're doing and get the hell out of the way. Even if you do see it coming, your cursor may be in the worst possible place, meaning you'll need to get a handle on it and put it where you need it to be before you can begin to move where you want, and this is time you may not have.
Speaking of cursors, here's another great way to make a fool of yourself on a grand stage. You're frantically clicking and acting, and then the teamfight starts. All sorts of lights and explosion happen, you see something that you have to respond to, and you go to click and...ummm...where's the cursor?
A lot of people have complained how bad the “yellow glove” cursor is in LoL, and how it's not readily changeable. If it were me, I'd like a bright, hot pink X, or some other obnoxiously loud symbol that I can't possibly lose track of if I tried. Maybe they'll let us do that in the client some day. Unfortunately, that's not really an option in the short term.
Solution: Short-length clicks with higher APM.
I have a friend that plays Dota and complains about us LoL players all the time, but there's one point he'll concede: controlling your champion in our game is so much easier than his. In his game, clicks are commitments, in ours, you can cancel almost anything.
So, you should be developing a movement style that naturally produces more clicks than needed. The more, the better. After all, you already spit out plenty of junk move commands in lane anyway, what's a few more? Here's the chance for that untrained middle finger to shine...get it to press the button more often.
The reason why is that clicks are NOT commitments, you can always take an auto or click to move back if it was made in error. By having a naturally faster clicking rhythm, your brain will not only get used to seeing your champion do the wrong thing (like a walk fault), and will already be operating at the proper speed to spit out another, proper click to correct it. In other words, it's not about preventing walk faults from ever happening, it's about being able to immediately correct them should you commit a click error. So, don't be lazy. You protect yourself from errors with a higher click speed, meaning that your opponents will have fewer errors to punish you for.
(BTW...lots of luck, Kalista. You and your “misclick to die” passive looks really dangerous to use in high level play...)
With regards to skillshot dodging, a high-APM style is invaluable, and even better if you naturally keep your cursor close to your champion. By doing this, you can change your champion's direction at any time, which is what you'll need to make sure that Morgana Q safely goes past.
To practice this, get a friend to lane against you in a custom game, and give him a skillshot champion. On your end, take some white masking tape and cordon off a square in the center of your monitor, big enough for you to move in any direction, and thin enough not to interfere with the display too much (I took one three-inch piece of tape, and cut it lengthwise into 4 narrow, three-inch strips). Lock your camera and lane against him for a while, only removing your cursor from the box when last hitting or firing a skillshot. Once you get used to this “fast and near” style, return to your unlocked camera and play normally. If you're using your ears to hear it coming as mentioned above, you’re all set and ready do decide which way to dodge and to dodge it.
Finally, the “fast and near” style will make it much less likely for you to lose your cursor. After all, it's near your champion. If you make a habit of returning your cursor to your champion after every time you take a significant movement (a camera adjustment, a long skillshot, etc), you'll always know where it is, and you won't have to waste any time looking for it.
In another of my attempts to pick a champion I can dominate games with, I recently gave Draven a try. If you can click correctly you'll wreck anyone. So I got into a few team builders and gave it a whirl. Naturally, I fell flat on my face. Simple laning took much more effort than I was used to, and when it came time to actually fight, I was making a fool of myself. I lost my cursor constantly, walk faulted all over the place, and generally embarrassed everyone who watched me. I've since returned to good ol' Caitlyn as my go-to ADC pick.
Naturally, this was a while ago. When I got used to playing Caitlyn more and I ironed out my kiting style with her, playing Draven with friends later became a cinch. What it came down to was using one of two kiting methods in different situations.
Solution: Rapidclick kite when attack moving forward, Attackmove kite when moving back.
“Rapidclick” kiting is the version where you just right click everything very fast. Right click the target to shoot, move the cursor, right click the ground to move, move the cursor back to the target, right click to shoot. This is not that hard if your mouse movements are small. If your mouse movements are large, you're begging to commit walk faults. So, it's easy when the target and the direction you want to move are the same direction; click the target, then click right next to him. Easy peasy. But, if you're running away, this style of kiting is too risky.
Instead, you can Attackmove kite when tracking backward. You keep the cursor in the same place, the direction you want to move in. When you want to shoot, shift click, a-click, or whatever you have the “attack move click” bound to in your key bindings. This minimizes mouse movement, thus minimizing the risk of a walk fault or a lost cursor.
In other games, “mechanics” means something else than it does here. In fighting games, it means recognizing when the opponent can't do something because of a particular choice they made. For example, Street Fighter 2 (Super Turbo)'s Zangief's spinning piledriver's move command starts with joystick movement AWAY from the opponent, a little more joystick movement, then pushing a button. If Zangief is walking TOWARD the opponent, how can he start the spinning piledriver? The joystick's in the wrong place. He'll have to move it to back to start it, meaning he'll stop walking forward, and you'll know it's coming.
In this manner, make sure you don't saddle yourself with Zangief's problem. Clicking tempo is much easier to control than mouse movement, so don't put yourself in positions for your cursor to be in the wrong place. Use the “fast and near” style in general, and make sure you use the right kiting style at the right time.
As an aside, there's a caveat about attackmove kiting: if you're running away from multiple targets, you may end up shooting the wrong one. That's a problem. In this case, you'll have to rapidclick kite. Good news...that's the next section!
RAPIDCLICK KITING, REACTION TIME
Solution: Play fast clicking games like this one.
Man, dat search bar.
Anyway, mouse movement, muscle memory, and rapid clicking are all learned skills. You weren't born with them, there's no evolutionary reason to have them, and you're not going to get them without practice. So...go practice. The link provided is to a flash game that you can play to practice. Write down your scores, and try to improve them.
As a side note, the reaction time version of the game is a little odd. It'll make you react to a stimulus with no warning whatsoever. That's not of very much use to us in LoL, as we know the champion we're up against, know what they can do, and can use tells and other information to prepare us for when we finally have to react. I tried to find some anticipation games, where you get a score of reaction time, but you're given warning of when the signal begins. Instead, try using a stop watch, and try to stop and start the clock on XXX.00 seconds. You can see which direction you miss in (don't ever do it early), and see how far late you are.
Well, that's all I got for now. Here are the tools you'll need to get ahead. I know it's the last day of the season, so if you have a ranked goal to hit tonight, go for it. But, come tomorrow, if you're still just queuing for games and not bothering to practice the skills developed here...well, it's your ranked career. You're the one who has to live with it.