Well.. I bet the only reason you'r here is to learn to play this biatch. Really i recommend just trying out some different builds and find the one that you like the most. Hope you enjoy it..

Always kill something!
As Yi its important to always hit something, really if theres no champion just go for a little jungling.
Early game:
In early game i like to play him aggresive, just go for the champions. If its someone like Mundo or Malphite, just nevermind and go for some lifesteal and kill minions. Just go for the items i listed and always hit something, sometimes you should be saving your double strike for a gank. If theres a jungler on the other team, sometimes hit the jungle and look for a dude with low hp.
Mid game
When you hit 10 or 11 you should fall back and just rape some minions. If you have like 20 kills, just keep on going with the destruction. You can also go for abit of jungling.
Late game:
This is where you will be raping, but still be careful a stun or slow will simply destroy you. In the team fights just stand back for a moment and then join the chaos. A good rule to remember is Master Yi - Last one in, first one out.
End game:
When you are Master yi. There is no such thing called end game! Even you alrdy won or have they surrendered.

Always use your Ult!
Always pop your ult! When ever your in a fight just pop it. Every time you get a kill it will refresh and since you are Master Yi you will always get the kill...
Youmuu's.
This is the same as his ultimate, just pop it when you can. Some times you can use this to killsteal. Its okey to killsteal as Yi! Assists is strongly publicized!
Summoner spells.
Really this is the same as the two others. POP IT WHEN IT CAN BE POPPED!

Thanks for your understanding. This is my first guide so don't overreact. I just want to show my point of view on Master Yi. I hope you liked it! xD