You start by trying to invade enemy topside jungle, because most players in low elo do not do early vision. Clear 2 of their camps, go back to your side and take your buff.

At this time the enemy jungler has probably noticed their buff is gone, they will either try to gank a lane or go steal your buff. Ping your laners to fall back and head for your other buff. With your laners' help you can beat the enemy jungler if he comes, unless he is one of the threats posted.

Only gank lanes that are pushed in, because low elo laners are bad at responding to ganks and usually fail unless the enemy is already pushed in. Never towerdive because you will always int if you do.

Red Start: go Red -> Krugs -> Raptors -> Wolves -> Blue -> Scuttle or Gromp
Blue Start: go Blue -> Gromp -> Wolves -> Raptors -> Red -> Scuttle or Krugs

By 5-6 minutes, you should get your first back and hopefully have enough for a recurve+control ward. Maybe even some more items. If enemy does not have dragon vision, sneak it, since Yi can take drags really fast. Ping your laners to let them know and pay attention to lanes so you don't get caught.

As Yi, you just powerfarm and carry your team's *** after 3 items. Mute all because your team will complain about not getting ganks while they overextend without any vision and get ganked 3 times in a row.

In teamfights, you want your team to engage first, let the enemy team blow their CC and damage on your tank, then go in to cleanup. Try to keep autoing, stack up rageblade and prioritize alpha striking the ADC and targets that have the highest damage potential. Don't chase the adc if you have other targets to auto, though. Getting kill is important as it resets your CD, and keep on attacking anything you can to alpha strike.