Hey! Im Snèakyfast. I have been playing Master Yi for awhile now with roughly 400k (and climbing) mastery points. Im Platinum on the NA servers and am around the 800th best Yi NA. I would like to keep this build short but informative so that it is a quick read while being able to understand mostly everything about how to play Master Yi.

PROS
Hyper carry champion
Many resets allow him to be a slippery champion to catch in a teamfight
Easy to get a hold of
Fast jungle clear with great sustain with W
One of the best 1v1 champions in the game

CONS
Hard to master the utilization of Q as a defensive ability
Extremely vulnerable to hard CC
Can get caught out easy if not careful
Extremely critical on when you enter a teamfight, its either hit or miss.

Conqueror: Best rune on Master Yi. Allows him to 1v1 anyone from squishies to tanks.

Triumph: Always good to have health back on a kill for clutch situations.

Alacrity: Attack speed on Master Yi is great! Is there any other reason?

Coup De Grace: Perfect for executing enemies to get your reset faster.

Nimbus Cloak: Synergizes PERFECTLY with Master Yi's ult + Celerity for an extremely quick gap close with a massive burst of bonus AD for a few seconds.

Celerity: Allows Master Yi to get a bunch of bonus AD from Nimbus Cloak and Ult.

BOTTOM SIDE: Aggressive Start
1. Allied Red Buff
2. Enemy Wolves (Be ready to kill enemy jungler as they might start botside)
3. Bottom Scuttle Crab
4. Look to gank bot/mid but dont force
5. Allied Wolves
6. Allied Blue Buff
7. Allied Gromp
Feel free to do more clearing but after the initial clear there is no particular order on where you need to be. Just get a feel for where the enemy jungler is and be ready to counter gank his ganks or gank a lane opposite of their location.

BOTTOM SIDE: Normal Start
1. Allied Red Buff
2. Allied Wolves
3. Top Scuttle Crab
4. Allied Blue Buff
5. Allied Gromp
6. Allied Raptors
7. Allied Krugs
Feel free to do more clearing but after the initial clear there is no particular order on where you need to be. Just get a feel for where the enemy jungler is and be ready to counter gank his ganks or gank a lane opposite of their location.

TOP SIDE: Standard Start
1. Allied Red Buff
2. Allied Wolves
3. Bottom Scuttle Crab
4. Allied Blue Buff
5. Allied Gromp
6. Allied Raptors
7. Allied Krugs
Feel free to do more clearing but after the initial clear there is no particular order on where you need to be. Just get a feel for where the enemy jungler is and be ready to counter gank his ganks or gank a lane opposite of their location.

When it comes to teamfights, Yi plays the role of the "Janitor". Waiting to engage AFTER the initial engage when most of the CC is down and the enemy team forgets about you. Master Yi can quickly burst down the carries/squishies while obtaining resets to stay in the fight and finish off the tanks/bruisers.

KEY POINTS IN HAVING A SUCCESSFUL TEAMFIGHT:
1. Prioritize the enemies main damage dealer or healer support. (Janna, Soraka, Nami, Sona, Etc.) You can ignore this rule IF there is someone very low that you can get a Q off on to obtain a reset and then go onto the damage dealer or support.
2. Go into someone who you are 100% sure you can kill and obtain a reset off of. Once you get a reset you will blowup out of control for the enemy to handle with how slippery you are.
3. Keep note of the summoner spells or active items used so you don't get baited (ZHONYAS HOURGLASS, Redemption, Heal SS, Flash SS
4. Find an opportunity to where the enemy front-line has used most of their abilities to get right to the back-line.
5. USE MASTER YI'S Q DEFENSIVELY OR AS A GAP CLOSE. I cannot stress this enough, if you are using Master Yi's Q for pure damage in a team fight with no utility purpose, then you are not playing Master Yi to the highest potential in a team fight.

If it is impossible to team fight, then you must resort to split pushing and rely on your 1v1 powers to apply pressure to side lanes.

KEY POINTS IN HAVING A SUCCESSFUL SPLITPUSH:
1. Keep an eye on the map, if you having more than 3 people coming to you, then tell your team to take advantage of that and they can apply pressure or take dragon or possibly even baron.
2. Know your strengths and weaknesses, Yi is an extremely powerful 1v1 champion that will win 95% of 1v1s if played properly. You must stack your Rageblade and have conqueror either close to being up or up.