My name is MeisMoxs, feel free to add me. I know what I'm doing with Master Yi ;) How can you tell? I have 200+ games with him, and am top 3%, and yes I am Mastery 7 with him, obviously. But all that aside, do feel free to give me some feedback because this is my first guide. And if you would like to see guides on any other champions, feel free to let me know as well.

Pros-
High Damage Output
Easy to Play
Cheap to Purchase
Great Pressure
High Threat
High Survivability Rate (with life steal)

Cons-
Squishy
Focused in Battles
If CC'd, gg'd

I know that a lot of Master Yi player take Lethal Tempo for their runes, but let me explain why I don't.
The first reason being, Conqueror does not have a time limit, unlike Lethal Tempo. Conqueror will last as long as you keep damaging an enemy. The second reason is that at full stacks, Conqueror you for 8% of the damage dealt, which helps boosts the life steal you already have from Legend: Bloodline and your build. The third reason being that at full stacks Conqueror also deals true damage, which will boost the true damage that Yi deals from his Wuju Style (E). Lethal Tempo does allow you to go above the attack speed cap, but only for 6 seconds, with the full stacks of Rageblade and Highlander (R) activated, you will almost have the max attack speed, so that won't be a problem.

Presence of Mind is a good choice since it'll Highlander (R) to be up much quicker, allowing for more damage output and healing potential.

Legend: Bloodline boosts the life steal that Yi already receives from his build.

Since you will probably be the main focus in team fights, Last Stand is the best option here. It allows more damage, which means more healing, which means even more damage, you get my point.

Now onto the secondary branch: Domination

Sudden Impact will boost Yi's damage for 5 seconds after he uses Alpha Strike (Q).

Yi does massive damage already without Higherlander (R), but with it, he does more >:D that is why you'll want it up as soon as possible and as often as possible.

Even though I said that Master Yi was easy to play, it does not mean that he have some skill-based things of his own.

Passive - Double Strike
Every 4th basic attack, Yi will strike twice, hence the name. The main thing you need to know about his passive is that it reduces the cooldown on Alpha Strike(Q).

Alpha Strike(Q) - This ability is THE most important ability to learn and master, if you want to main Yi.

Dodging - It allows you to dodge cc which, almost always results in your death. Use it to also doge high damage abilities, like the ults of Darius, Garen, Neeko and Miss Fortune. It also allows you to dodge tower shots, if timed correctly, so feel free to dive with no worries. Because if you Alpha Strike(Q) under tower, it will re-prioritize back to minions.

Chasing - It also allows you to stick to enemy champions when they flash, dash or blink. This is great for when an enemy champion flashes across a wall and you're still right there beside them.

Meditation(W) - Meditation reduces the incoming damage by an AMAZING 60% AT LV 1! At max LV it is increased to 70%, if you time this correctly, it will allow you to survive many, many different situations. Please keep in mind that the damage reduction also works on tower, so like I said, feel free to tower dive ;D

Wuju Style(E) - Use this only AFTER you use Alpha Strike(Q), it does NOT boost the damage of Alpha Strike(Q), and will only shorten the duration if you use it before.

Highlander(R) - If you use this when ganking, you won't need to use Alpha Strike(Q) to chase since Highlander(R) gives you a movement speed boost. So save that handy ability for another time. Highlander can double if not triple the amount of damage that you dish out. "Great power comes with great responsibility" - Uncle Ben

Surviving - Please, please, please keep in mind that, even though Master Yi does tons of damage, and can survive fighting under tower with life steal, he still is an ASSASSIN, meaning that he is squishy. And since Master Yi does so much damage, remember that you WILL be targeted in battles, so TRY to not go in unless the enemy team has use all of their abilities, or at least the cc ones. And when it is time for you to go in, FOCUS the people that have crowd-control abilities, since you WILL win every single fight against everyone else.

That is everything that I know about Master Yi, what works best, and what to look out for, 200+ games of experiences condensed into 1 page. What you do from here on out with that information is not in my control, but please, wreak havoc wisely ;p