yi is all about farm farm farm, u have to be ahead to be relevant, but u also have to gank lanes, drop wards, and i guess with this new patch kill the scuttle crab every 2 mins or you lose.
late game u cant really engage without the enemy team's hard cc's down, u have to let ur team defend 4v5 while u split, or be ready to flank as soon as they drop all their hard cc, and focus down the dmg dealers first.
usually my yi build is as follows.
Then usually some defence (this is usually my build, but these 2 items can be and should be switched to best counter the other team, or just mass uber dmg if u are super ahead, then sell late if u cant delete all the dmg dealers quickly and get a little more tanky for a teamfight)
basically your main kit includes but is not limited to,
your jungle item (skirmish is best so u can add extra dmg instead of slow)
BOTRK
Rageblade
Then the other 2 items besides your boots should be for defensive purposes, or for better tower taking abilities.
I am also known for using 
use it the same time you use your ult and its pretty devastating.
but the best thing u can do is find out who "Cowsep" is, watch him, and learn.
i got to plat 5 back in season 6 just by watching and imitating him, you can learn more about yi from him than prolly anyone.
GLHF, i hardly play yi anymore since its so hard to get ahead if ur team just loses lane again and again even if u gank, so that leaves u with 2 options, split push and win, or split push and lose, since u will be hardly useful in teamfights if the enemy team is ahead of you.
