Okay got a bit of time, I am going to try to write this out quickly so there might be some spelling and grammar fuck ups! :D
Alright I am going to start with a disclaimer that I am nowhere near Cowsep's level of Master Yi play, I still consider myself a Mister Yi player. I am only in gold currently and my MMR has me mostly with people that were high gold, plat, and low diamond last season.
Also I'll just preemptively explain that yes, Yi has a very low skill floor but also a surprisingly high skill ceiling do to the insane amount of game knowledge and timings he takes to play at Cowsep level.
Alpha Strike
Main reasons to Alpha Strike, try to save it for these situations. It isn't just there to do damage.
Some tips.
Especially late game, your Q will actually drop your DPS vs just AAing so if you use it as a normal damage ability its actually hurting you. If you have used your E you will lose a bunch of true damage by Qing
You want to try to avoid using it to clear waves while split pushing. you are vulnerable any time it is down.
Always prioritize dodging hard CC over all other things.
Your Q is longer the more things it hits. This means you can more easily dodge non targeted moves. Also it makes it incredibly easier to dodge zed's ult if you around more minions. This also means using your Q around more people will significantly drop you single target DPS so again use it for plays, it is not a damage ability
Always fight zed near minions. you cannot dodge the start of his ult, you can only dodge the application at the end. The more minions are around the less precise you need to be about the dodge.
If your initial Q target dies before you finish, you go all the way back to where you were when you started. This very rarely come in too handy outside of farming/poking in aram. Say an enemy is about to die and is around minions, you can Q them through a caster minion and smite the minion mid q to jump all the way back to safety when it finishes. This opportunity is rare but holy crap does it feel badass.
Remember to AA nonstop if Q is on cooldown. trying to duel someone around minions and they zonyas? farm up so you can dodge the rotation that comes out when they get back.
Meditate
Your W is relatively self explanatory. You use it to block heavy damage, heal between jungle camps, and as a stall in combat if you pretty much fucked.
Some tips.
When blocking a single attack with it, be as precise as possible. You are just using it to reduce the chunk, not to heal. the longer you use it the less damage you are dealing and you would probably be healing more from your lifesteal.
use it before you potion in the jungle. It heals more when you're lower, If you are trying to heal, use it before you have a chance to recover from other sources in order to maximize it.
If you dove and are stuck looking at a golden statue, meditate while you wait if you cant AA minions to reset your Q.
Wuju Style
Use it while fighting, try to not waste up time by Qing unless you have to. This shreds tanks to hell.
While it does scale with AD, it makes him scale even better with AS. one of the few champs that can rush AS before AD and still deal shit loads of damage.
keep track of how many AAs you can do during its duration and make sure you maximize the number of double strikes you do during it. Especially in the early jungle and when fighting tanks
Highlander
Try not to fight with out this available.
Don't go near anyone that can slow your AS without this. Frozen heart and Randuins will wreck you. if this is down, farm your jungle.
General tips
You need ridiculous game knowledge. You need to have the animations of most champions memorized if you want to be able to dodge or block them. Go to the model viewer and learn their tells.
You need to learn how your current opponents play. Not everyone is going to play the way everyone else does. Your first couple fights will show you how they do their rotations and what their person tells are. These will help you dodge/block instant cast abilities like tibbers or variable duration abilities like nunu ult or jax stun.
You can solo dragon at 3:30. Even with the nerfs to jungle items you can still do it. though not being able to pick up the temporary rangers makes it less safe. everything just needs some serious dodging of aas.
When fighting dragon you need to dodge and block as many of its attacks as possible. Try to stand directly under it. The further you are from it when you Q dodge the higher chance it seems to have for it to do its mini reset which heals it slightly and makes it go down and jump back in the error which pushes you away. Honestly I still have not figured out exactly how its reset works...
Getting dragon that early lets you help all your lanes from the get go. which is good since your early ganks are kinda shitty...
End
So that is alll i can think of right this second. I would recommend watching Cowsep if you want to actually learn better than I can explain. Or if you really want, I do stream myself a few times a week and I can explain stuff better there then here. I don't many viewers so its easy for me to interact with them when I do have a few. But yeah Cowsep is a way better option