(remember to read notes on abilities, at the ability sequence)
Yi is a fabulous jungler, he has the ability to clean packs with ease using his Q and mana free E. This is why he mkes more gold in jungle than on a lane. He is fast, and packs a lot of damage, making him capable of moving fast from one lane to another if necessary. He has a gap closer. So he is useful for ganks !
You start machette and potions because the jungle will damage you a lot, and sure you have meditation, but believe me, you WILL need those 5 hp potions.
The ward is to help a lane or secure your buffs in case the ennemy shows up.
DO NOT gank before your lvl 6. DO NOT gank at lvl 6 without a red buff. In any other case, you will be almost totally useless.

Once you back at 1300 gold, you can buy the spirit stone to have infinite mana and a sustain in jungle, and the boots give you needed mobility and attack speed (which helps stack passive and helps get your Q faster.
Time to farm and once second red buff appears to try to gank if it is worth it. (dying for a kill is never worth unless its an interruption and you are not one)
Farming is the key to winning mid game.

HERE IS WHY I DO NOT RECOMMEND FERAL FLARE.
First of all the item is expensive, 1800 gold. Then it gives only 15 ad, and a tiny bit of attack speed, the sustain is weak and no mana sustain, and most of all the flare damage once it's made is MAGIC. IT DOES NOT CRIT !! Yi with this build is made for insane crits, this does not benefit him at all. And also, the base champ damage is 48 (33 from the flare and 15 ad) knowing the armor and mr lower it. You gain 1 damage on champs per big creep, and its magic. No crits. so... No flare on yi.
Same for lizard thing, no crits on the true damage it gives. And you already clean the jungle in seconds, who cares of the tiny damage over time it gives, the red does the same thing but it SLOWS that's why its usefull.
the golem one is for tanks, yi is kill or be killed, he is not a tank. And the other one is ap that says it all.

Ok now you finally have infinity edge. With the bonus attack speed of your R and the bonus crit on the runes, you should be able to dish out nice damage when you gank. It's always fun to do 350-450 crits on people with 1300 hp. Try to gank if usefull as usual, and FARM !! FARM IS LIFE !! PRAISE THE FARM ! FARM TO OBLIVIOOOOON !!! (Sorry... thats what happens when you play yi too much).
If you get the phantom dancer, you will find that the ennemy players usually seem like paper sheets to your blade.

Also since you are a heavy mobile dps, your strategic importance is high enough that you are allowed to take kills.

Ok you've got IE and phantom, now you've just acquired yuumu. Just imagine this. you have 80% crit. You have lvl 2 ult usually at this point. Yuumu + ult is 35% + 20% total 55% move speed and 55+40 total 95% attack speed bonus. You WILL do huge amounts of damage. The problem is knowing when you can dive in. You do not need to be visible, if you add ghost you have 83% move speed, its tremendously high, you can come from a bush somewhere nearby. if you get stunned or bursted : you die. If you don't : ennemy team gets laminated in SECONDS. Yes seconds. Like 5 seconds. BUUUUUT you are fragile so... Use your brain before dashing in.

You will need to know where your ennemy is before he knows you are there. A suprise attack usually gets a kill. Here's how it works : stay in a bush bronze trap style. When an ennemy facechecks like a victim land two auto attacks on him, that s usally the time before he flashes or something, the brain needs those seconds under suprise. That's when you Q immediately so you catch up or evade a skill. And there you go. Dead meat.
Also remember that once you have last whisper you should be capable of soloing baron, it sometimes is useful.

A yi is supposed to kill or be killed with this build, if you don t kill in less than 2s then you either failed in your engage or gameplay, or your team failed. 99% of the time it will be you, not your team.