If you choose to jungle as y (i assume you are since you're checking this build) you should get the blue buff first, ask your teammates to help leesh, it will spawn at 1:50 once it is at half health your teammates should leave and you smite it for the kill, then kill the two minions next to it. you should now be level 2, now go kill wolves, wriaths, golems and you should be level 3, you can now kill the red buff by yourself if you alpha strike and smite it to kill.

At around the 5 minute mark you should be a high enough level to help your teammates with a gank on the top, bot, or mid for a kill, but be sure to talk to them in the chat and don't just go in. after the gank you can either push the lane or go back to jungling.

In teamfights the best way for master Yi to help is to kill steal. once one of them is low on health you hit your ult(R) and alpha strike them, because you killed them your ult resets the cooldown of all of your skills and now you can hit it again and alpha strike for another kill, and repeat until they have backed off, all died, or you die, or your team fell back, as a final protip, with your heal(W) scaling off 400% ap it will recover insane amounts of health and increases your armor, it will be able to save you if you hit it and the enemy is attacking you, unless you get knocked it the air or silenced. you are also going to be very good at backdooring which is what the teleport is for(you can teleport to minions) and use your alpha strike(Q) on enemy minions to take the waves out super quick and push the lane.