This is a guide for Master Yi for games on the Twisted Treeline. This is my first guide, so it won't be perfect, but I have had great success with this build. I build Yi as an AD/AS carry, able to dish out lots of damage in a small amount of time.

Master Yi is a Melee DPS carry who specializes in wrecking squishies, farming, pushing, backdooring, ganking, and much to the annoyance of my teammates, stealing kills.

Master Yi has a few runes that he must have, and some others that are optional.
Greater Mark of Desolation is great for the armor penetration that it provides.
Greater Seal of Attack Speed gives some attack speed, which is wonderful for doing more damage in shorter time and last hittting minions.
Greater Glyph of Magic Resist doesn't seem like much of a Yi rune, but since magi can nuke you very easily, having the ability to stop that is good.
Greater Quintessence of Desolation gives even more armor penetration. With these quints and marks, characters without piles of armor will find that you practically deal true damage to them.

Getting 21 on offense is pretty regular for Master Yi builds. I split the remaining 9 points among defense and utility. Obviously, you want to spec heal if you bring it. In utility, health and mana regen are good, plus extra XP and less time spent dead.

The spells I usually bring to the Treeline are these:
Heal Most people laugh at this one, but it has saved me so many times from 1v1's or diving that I always use it.
Ignite I use this even more than heal, because you can get quick kills or ks's. Also, it can stop Dr. Mundo's Maximum Dosage and other healing moves in their tracks by dropping its effectiveness by 50%.
Spells I don't use:
Revive No. Just....no. You shouldn't die that much anyway.
* Clairvoyance An okay spell, but there are much better spells. Besides, you don't need to know where the enemies are, just how to kill them. (That was a joke, by the way)
* Clarity Master Yi isn't mana hungry enough for this spell.
* Exhaust This would probably be in the top 3 spells for Yi, but I just don't use it. Use it if you want though. NOTE: I have been using this more often, and it is often better than heal. Since Yi has no CC, he can catch people better. I won't actually replace it in the top of the guide, but I just want you to try both, see which one is better.
* Ghost A fairly good spell, probably one of the better ones for Master Yi. However, Highlander covers for this area.
* Cleanse Since Master Yi is so squishy and prone to disables, this would probably be one of the other top 4 spells for him, so you can escape ganks.
Smite Are you jungling? No. No smite.
Fortify Not a 3v3 spell. They will be too busy getting slaughtered by you to push your turrets.
* Flash An okay spell for escaping or ganking, but there are other better ones. You can use this one if you want.
Teleport You are fast enough that you don't really need this that much.
Rally This is a spell? Wow. I've never seen it used before. Anyway, it has too long of a cooldown to be useful.
*Not forbidden in the Book of Yi.

At the beginning, buy Boots and 3 x . Then run as fast as you can to the bush at the bottom. (Unless you want to either skip the gank or gank somewhere else) If you go to the regular bush, you should get in it as it says "Welcome to the Twisted Treeline". Wait for your team, then start attacking the squishiest one on their team. You might be tempted to use Alpha Strike to initiate. DON'T. Go in, hit them a few times, then use it if you need to mop up or get a ks. Many people get the Vampiric Scepter as their first item, but I personally don't like that choice. Most of the time, I am able to stay in my lane until I have enough for Berserker's and the Vamp Scepter. Also, without Boots of Speed, you can't chase down kills.
Master Yi is a pretty good top laner, so you generally want to take that, unless you would be up against someone you can't harass, or a nuke like Brand. Get last hits as much as you can on minions, and harass the opponent. Using Alpha Strike on a minion when there are 3 or less and the enemy champion will maybe kill a few minions and hit the champion. If you do this a lot, it will annoy your enemy so much that they will stay out and let you outlevel them. Hopefully. If they are a squishy ranged carry like Ashe or Caitlyn, they will probably harass you a lot, so you need to actually get close to them to kill them a few times. But first, you need better items. When you hit 1020 gold, recall and grab Berserker's Greaves and Vampiric Scepter. Run back as fast as you can so you can keep getting tons of money and XP. Ganking low health bottom laners is a great idea at this time, when you might be a level six so you can use Highlander.
Keep doing this until you can buy the rest of Zeke's Harbinger. If the team has at least one tank, buy Madred's Bloodrazor to wreck the tanks. If their team has a lot of squishies, then Phantom Dancer is good for the critical strike and attack speed. After you buy one of those, then buy the other. If the game isn't over by now, either you are doing something wrong or your team is horrible. But, in the rare circumstance that the game is not over, Bloodthirster and Infinity Edge can do wonders with extra damage, lifesteal, and critical stuff. You might need some armor penetration, so replace something (probably Stark's) with Last Whisper. By now, anyone should die to you in less than 4 seconds, and towers should go down just as fast.
Here is the part that a lot of people complain about: They say I don't put enough damage into my build. I disagree. Wuju Style gives me 70 damage when I activate it, along with Madred's. (I'm not counting the last two because I rarely reach those items). The attack speed makes me do that extra damage very quickly. Also, Critical Strikes essentially do 100-150 extra damage every third hit. So that is around 50 extra per hit at later levels. If I find a great item to sub into the build, I will, along with an explanation of why I used it. (Note: Cloak and Dagger is a good item, but I can't seem to add it. I'll keep trying, though)
EDIT: Madred's Bloodrazor is good for heavy health tanks, but I've pretty much stopped getting it on 3's. It costs a ton, and it doesn't do enough against regular enemies. A much better, barely more expensive item is Infinity Edge. However, if the enemy team is heavy on bruisers (ie, Mundo, Garen, etc), it might become viable. You can always get it at the end, too, so you wreck stuff even faster when it matters the most.

There are a lot of items that have great synergy with Master Yi, but, alas, only 6 are allowed the honor of being used by him. Some items that didn't quite make the cut are shown here:
OFFENSIVE:
Cloak and Dagger When facing teams heavy on CC (like Blitzcrank and Amumu, etc), this item helps solve that while giving AS and Crit.
Youmuu's Ghostblade AD, Crit chance, ArP, CDR, all in one neat little package. Also, a mini-Highlander active. However, the amounts of each on this are rather small.
Guinsoo's Rageblade Not the greatest item unless you are playing hybrid Yi, but still pretty nice with its passive (stacks of AS and AP)
Malady and Nashor's Tooth are both like Guinsoo's in that they are good for hybrid Yi, and Malady applies an onhit effect.
Stinger without going all the way to Nashor's is pretty good, AS and CDR, and fairly cheap.
Sword of the Divine Straight attack speed with a small onhit effect. VERY good against Jax.
Black Cleaver Lots of AD, a little AS, and armor shredding. Good vs. Amumu, Malphite, Rammus, etc.
Wit's End Give you magic resist stacks, which is useful vs. magic heavy teams.
Frozen Mallet A VERY nice permaslow, and some health and damage. Fun to get if you're playing on a smurf.
Hextech Gunblade Good on hybrid Yi, and somewhat good on regular Yi.
Last Whisper Some AD and some precious ArP.
Manamune I don't usually have huge mana problems, but if you feel that you do, get mana seals (tier 2 if you want, there's not a huge difference)
Sword of the Occult If you think you will get REALLY fed, grab this. If not, it's a waste of money.
Tiamat Splash damage is cool.... but not extremely useful. Only in customs/ smurf games.
DEFENSE:
Guardian Angel If you are getting mauled and want a second chance, here you go.
Banshee's Veil Blocks spells, give health and mana, and MR. Good item vs big time casters.
Quicksilver Sash Gives MR and a free clease ever 1.5 minutes (as of patch v1.0.0.120)
Warmog's Armor If you just want some solid health.
Thornmail Only if you ABSOLUTELY need it. Against, say, Tryndamere.

Yi has a wonderful set of abilities.
Double Strike Is his passive. Master Yi strikes twice every 7th attack.
This is a great passive, because you can time it to do lots of extra damage. You know that this is up because the icon for it changes into a purple sword cleaving some stick figure in half. Pop Wuju Style before you use this on a champion to deal an extra 140 damage in that one hit.

Alpha Strike This is his Q move. Master Yi leaps across the battlefield with blinding speed, dealing magic damage to multiple units in his path with a chance to deal bonus magic damage to minions.
Master Yi leaps across the battlefield striking up to 4 enemies, dealing 100/150/200/250/300 (+1 per ability power) magic damage to each enemy with a 20/30/40/50/60% chance to deal 400 bonus magic damage to minions.
Cooldown - 18/16/14/12/10 seconds
Cost - 80/90/100/110/120 Mana
Range - 600
This move is a great initiator, because it pops you up right next to your enemy. Also, this is a good farming tool at later levels, when almost every minion you hit dies instantly.

Meditate Yi's W move. Master Yi rejuvenates his body by focus of mind, restoring health and increasing his armor and magic resistance for a short time.
Master Yi channels, restoring 140/280/420/560/700 (+1.65 per ability power) health over 5 seconds. While channeling, Master Yi gains 100/150/200/250/300 armor and magic resistance.
Cooldown - 35 seconds
Cost - 70/85/100/115/130 Mana
No range: only works on self.
This is a great move for AP Yi. I don't have a guide for that build, but you can find one somewhere if you want to try it. This move heals you for a good amount while making you very hard to do anything to at later levels. Once, I tanked Tryndamere while he was on Undying Rage with this move. So it's pretty good. Especially when you are next to an allied turret.

Wuju Style Yi's E move. Master Yi becomes skilled in the art of Wuju, passively increasing the power of his physical attacks. Activating Wuju Style doubles the passive bonus damage for a short time, but the passive bonus is temporarily removed after the effect wears off.
Passive: Increases physical damage by 15/20/25/30/35.

Active: Master Yi can activate this ability to double its effectiveness for 10 seconds, but this removes the passive bonus until Wuju Style becomes available again.
Cooldown - 25 seconds
Cost - 40 Mana
No Range: Only for self
This is Yi's main damage output. It has a very low mana cost, so you can use it practically whenever it is up. A very good skill when used in conjunction with Double Strike, so you deal lots of extra damage. It is best to leave it on the passive while farming, and to turn it on for champs and turrets.

Highlander Yi's Ultimate and R move. Master Yi moves with unparalleled agility, temporarily increasing Master Yi's movement and attack speeds as well as making him immune to all slowing effects. Additionally, killing a Champion refreshes all of Master Yi's cooldowns.
Increases Master Yi's movement speed by 40% and Attack Speed by 40/60/80%, and he becomes immune to all slowing effects for 6/9/12 seconds. Additionally, killing a champion refreshes all of Master Yi's cooldowns, and scoring an assist reduce the cooldowns by half.
Cooldown - 75 seconds
Cost - 120/120/120 Mana
Hoo boy. This is a great Ult for many reasons. First of all, you can escape very well with it. You get much faster and can't be slowed. Second, you can chase down kills. Third, it increases greatly how fast you attack. Finally, every time you get a kill with Highlander on, it recharges all of your cooldowns (except for summoner spells, of course). Use it usually for kills, escapes, or backdooring if you are losing and need to kill the turrets quickly.

I get Alpha Strike first for its ganking prowess and skill at harassing. After that, I alternate between Wuju Style and Alpha Strike, upgrading Highlander whenever possible. If you feel you need more damage, get Wuju Style more than Alpha Strike. If you find yourself always needing more health, invest a point or two in Meditate early. A lot of the skill point investment is reliant on how the game is going.

Farming as Master Yi is very very easy. His attack speed makes last hitting very easy, and critical strikes make short work of minions. Alpha Strike can take half of a wave if it procs on all 4 targets. There is not much else to say about farming besides not turning on Wuju's active on minions if you can help it. That is better for champions. Also, when you have Double Strike up (the icon changes), then you can try to hit an enemy champion for some double damage.

I see lots of mistakes while facing other Master Yi's playing. One major one is initiating with Alpha Strike at low levels. It has such a long recharge when it is low level that you can't use it again for a while, so the enemy can escape easily.
Another mistake is using Highlander to get TO battles. No. You use it when you are IN battles so you can kill people faster. That way it recharges instantly.
So here is how to do things right:
At low levels:
Run in. Pop Wuju Style. Wreck them. Use Alpha Strike to finish them.
Or: Run in. Pop Wuju Style. Get wrecked. Run away, maybe using minions to leap off of with Alpha.
At mid levels:
Run in. Pop Wuju Style. Wreck them. Pop Highlander. Use Alpha Strike to finish them off.
Or: Run in. Pop Wuju Style. Get wrecked. Pop highlander. Run, run, as fast as you can, you can't catch me, I'm the Gingerbread Man! (with a sword)
At High Levels:
Alpha Strike in. Pop Wuju Style and Highlander. Use Alpha Strike if you have to, and they are dead.
Or: Sorry. There are no alternatives. You will not die. Or need to run away. You are fed Master Yi, and nothing can kill you.

Master Yi is an ideal backdoorer. On 3v3's, backdooring isn't as common, but it can still be viable. If your team (a competent team) is in a fight with the other team, you may want to leave and kill a turret with some minions. This may cause a teammate to die in the fight, but turrets are permanent, so killing one is more important than a team fight. He can use Wuju Style on the turret, and Highlander if he needs it done in a hurry.

If you are losing the game, but you want to farm, feel free to go out. However, DO NOT GO BELOW 120 MANA!!! If you do, you will be completely vulnerable and unable to escape. Then the enemy will push your team, which is most likely a fail team, and you lose. So stay above 120 if you have any risk.

After you use Highlander and Wuju Style and get a kill with it and observe that you still are strong enough to pwn everyone else in sight, DON'T USE R AND E YET! Wait until they both run out. If you pop Highlander immediately, then you don't get as much time with it. Oddly, if you pop Wuju Style, your damage goes DOWN.

If you have an ace, are low health, and are killing a turret, hit a few minions periodically to restore your health. Even though they are all dead, if you lose track of time, they can pop out and massacre you and get 1000g (because you have so many kills).

Yi is not a total jungler on Twisted Treeline, but it never hurts to hit the monsters once in a while. If you are running by with full mana, feel free to use Alpha Strike on Wraith Camp and get 70ish gold. Getting buffed up with speed, attack speed, CDR, slow and true damage extra is wonderful for ganking or just plain killing.

I have done a game once where it was literally 1v3. Of course, this was on my alternate account. Secondly, I rushed Frozen Mallet, which was the core of my "cocky build". If you see a team that is particularly noobish, feel free to pick this item up for some absolute dominance.

Well, I hope you enjoyed this build, and have great success with it. Feel free to post comments or suggestions. But please try the build before you downvote it. If you don't like it, tell me why so I can fix it. That's what this is here for, after all. I just want to add, with this build, I have a 4:3 win loss ratio. Or something around that. Anyway, it works for me, and I hope you share my success.