Reading this part is not required to understand the guide, it just lets me explain some things.

1. This is a guide for an AD Yi, nothing else. I am not a competitive player, but I can still understand what to go for.

2. In my opinion (Just my opinion) Master Yi is a DPS master, but can also be taken down by high armor/ HIGHER damage DPS's so it will be covered.

3. This guide can be used by anybody (as long as you want to) and is not limited to unranked matches.

4. Any recommendations are appreciated.

This is a text-based guide. If you're not going to read all of it, don't come :(

Welcome to my Master Yi guild! If you have any comments/ recommendations feel free to leave them in the comment sections below.

What I see wrong with most people using Yi (who don't know what they're doing) is rushing into battle, head on, without thinking. You're a DPS, not a tank. The point for you is to be able to rush in, deal massive damage, then get the out ASAP.

When people rush into battle, they give a bad impression of Yi, hence the reason that he is rarely used in any sort of competitive battling. When mastered, he is beyond deadly.

This guide will not be covering jungling, so if you would like to know more about that, here is a nice video explaining it.
http://www.youtube.com/watch?v=-Lta-QiSCQw

Pros

-Huge amounts of burst damage
-Fast base speed (330)
-Great at ganking
-Great at farming
-Amazing pusher

Cons

-Very squishy early-game
-Needs good items to really shine
-If you fall behind, it is extremely difficult to get back up
-Large target

The masteries (as shown above) can be moved around to your preference, but I have found these to be the best. If you can't get all 30, here is the prefered order:
Deadliness x3- Good hands x2- Perseverence x3- Cripple x1- Sorcery/ Alacrity (interchangable) x3/ x2 respectively- Burning Embers x1- Sunder x3- Offensive mastery x1- Brute force x3- Lethality x3- Havoc x1- Awareness x4

Here is a bit of explination for some of the masteries:
Good hands: Well, if you unfortunately die, you will be back to kill sooner. More time alive + amazing damage = :)
Perseverence: The increase in health/ mana regen is helpful because it lets you stay and fight for a longer amount of time. Early game, it can allow you to stay in your lane for a longer amount of time, which gets you more exp/ gold for later on.
Scorcery/ Alacrity: These two masteries are interchangeable because, depending on what you want, you can go for it. As long as you have both by the end, you're going to be fine.
Burning Embers: The main use for this is that once Ignite is sadly gone, you will still have some use for it. The extra 10 ability power can help you with Alpha Strike, which will help you farm more and more. It just gets you more use out of an already nice skill.
Awareness: Once you have everything done, now you can focus on this, just because you want more exp. The sooner you get to level 6, the sooner you can start running around and ganking people. I saved this for last because there are just some things that take a higher spot in importance.

Now we get to the fun part; items. You want to start with Boots of Speed, just so you can get a bit of extra movement. From there, get either 2 Health Potion and 1 Mana Potion, or 1 Health Potion and 2 Mana Potion (depends on what you prefer).

Once you can, get Berserker's Greaves and start on your Zeal. Don't go back as soon as you can get the Greaves, make the most of your time there and start on your Zeal. If you happen to die, just buy everything for that you can, at the time. Make yourself efficient. Once you have all that, you should be around 6, so now comes the fun part. Stay in your lane for a little bit of time, and continue harassing with Alpha Strike, and when you see an opportunity, take up Wuju Style and Highlander AT THE SAME TIME and get the kill. Once that is done, recall and start killing lanes.

Now, after getting a few kills, finish your Phantom Dancer. This will give you vital attack speed, movement speed, and critical chance. All vital for your next big buy: Infinity Edge

WHOA WHOA WHOA, Infinity Edge this early!? Yes, I'm serious about it. Why? Because if you are doing well enough, you should be getting a pretty good flow of gold, so you should be able to get the Infinity Edge relatively quickly. Don't be afraid to buy it piece by piece, just remember to get it.

Now you have a ton of damage and attack speed, but there is something missing: Health. Get a Vampiric Scepter for some life steal. With your high damage, you should be able to get health back by killing minions.

The next part is getting Phage. It will give you some greatly needed health and the passive helps you chase down enemies. From there, you can either get Bloodthirster or a Frozen Mallet, whichever you think will help you more. Both are a great item to have, and will ultimately help you finish off almost any champ you may face.

Now you should have 5 slots taken. Now it gets strange, because you want another item, but there is only 1 spot left. What do you do? Well, the best thing I found is to sell your Berserker's Greaves. Why? Because the Black Cleaver will help you a ton. Now you still have one more slot, and now you face a dilemma. Boots, or some other kind of weapon. I suggest getting the Boots of Swiftness. Why? Passive speed 3 will basically make you a chasing machine. By the end, you should have:

-> -> -> -> -> .

Just reading that makes you sound like a killing beast.

If you ever need another item (situationally) just go with whatever you like. Some things I would suggest:
Banshee's Veil: Anti-CC
Quicksilver Sash: Anti-CC, plus Magic Resistance
Guardian Angel: Makes you not a target... Ok, LESS of a target.
Zeke's Harbinger: Ok... this is a bit of a stretch, but you always want to have somebody with this. If you can't convince somebody to get this, it's up to YOU!
Last Whisper: Against high armor enemies, this will tear them to pieces

Please (PLEASE!!! ;-;) don't use the following items, as tempting as it may seem:
Warmog's Armor: Oh please don't use this ;-;, you're not a tank.
Rabadon's Deathcap: You aren't going for ability power, you're going for attack power. It is useful for others, just not Yi.
Avarice Blade: I know how temping these gold items are, but for this, you have no use.

Assuming you aren't jungling (which if you're following this guide, you shouldn't) you want to invest in Alpha Strike first. With this, you can start farming quickly, and efficiently. Grabbing the blue buff will also help you with lowered cooldown and higher mana regen rate. Alpha Strike will also help you harass the enemy champ(s). By the time you reach level 6, you should be able to start killing champs, and who doesn't want that 300 gold. Now it is just a balancing act of killing minions, and champs. Just remember: Always be on the hunt!

Double Strike

Here is a passive that will ultimately be your savior, and your best friend many times. After 7 basic attacks, he gets to do a double strike, which is two basic attacks in one. Extreme damage + High Crit Rate + High AS = Win.

Alpha Strike

Here is your vital skill. You will need this from everything from ganking, to chasing, to evasion (I'll go into that later) and, most importantly, FARMING! I have a lot to say about this skill, so it will have it's own section later.

Meditate

Here is an example of a skill that is great, but gets no love, because of the character it is on. Now, that isn't to say it does have it's uses, it's just that your main job is to run around and kill. The fact that you can heal yourself for 700 hp without AP is great, but the problem comes when you look into what you are doing. You are killing, not tanking (hopefully ;-;) so this skill, although useful, isn't vital. Leave it for last, you may have some uses for it later on.

Wuju Style

Oh Wuju, how I love you so. Basically what this skill does is that it gives you increased attack damage passively, and when you activate it, the passive amount of damage is doubled. At level 5, it does 35 passively, and 70 activated. Yeah, think about that. Along with his ultimate, it makes a deadly combo that just can't be beat. Along with the fact that it gives a great boost, it can stack to some degree. If you have Wuju active, and passive at the same time, you have 105 attack boost. Yeah.

Highlander

Here is when Yi gets to finally shine. Highlander increases your attack speed and movement speed by 40/60/80%. Now, along with Wuju style, you have a crazy effective combo. It helps with chasing, killing, and just plain harassment. It gives you a huge burst of damage, and lets you get away if you need. This is just a great skill, defensively and offensively. Along with all that, if you kill an enemy champ, YOU GET ALL YOUR COOLDOWNS REFRESHED! How can you not love that?

Alpha Strike is probably one of the most versatile skills in the game. Why? Well, lets break it's uses down.

Ganking

: Well, Master Yi has great damage output, so it's only natural that you would want to do this. Nice job, you have good instinct when it comes to the game. Now, when you think about ganking, you think about jumping out, killing quickly, and getting the heck out. Master Yi can do all of that, and much more! Just Alpha Strike to initiate the attack, then go for Wuju Style and Highlander. You just killed a guy in 5 seconds flat. Yay :).

Chasing

: Now we have another use for such a skill. It allows you to jump to your enemy as long as long as it's in within a range in 600. Also, you can use it to turret dive (don't try it unless you are sure you can). Now, if your target is running faster than you (Strange, you have the highest base speed in the game), well, Highlander and Alpha will make sure you are going to catch, and kill, your enemy. 3 cheers for killing!

Evasion

: This is a harder skill to master, but it can still help. Now, Alpha Strike gives you a few frames of invincibility while you're getting to your target. If you can time this perfectly, you will be able to dodge basically any spell. Hard to master, but useful none the less.

Farming

: Here is when Alpha Strike shines. Along with it's base damage, of 300 at level 5, it gets a chance to get an extra 400 damage to minions. By level 5, it gets a 60% chance. Well, it's always nice to be able to one shot minions, and clear them out quickly. Don't you just love this skill?

Well... there is one downside. It is a major mana drain. You better be able to keep alpha striking, or you may be in trouble. That doesn't mean you have to leave, just means you have to camp... yeah. That's why you want to get a blue buff, or a couple of mana potions. After some time, you will be able to completely ditch all the potions and just get the blue buff.

Recommended:
: Allows you to damage your enemy, while staying back. I've actually gotten a few kills by igniting people who were hugging turrets.
: Allows you to slow your target, making your job even easier. Horray!

Possible:
Ghost: Possibly, but you are already fast. You can always just Highlander away.
Teleport: Well, useful, but not as useful as the others.
Flash: Same as teleport, not as useful as others.
Heal: Nice, but not needed.
Cleanse: Meh, not really amazing, but somewhat useful.

No-no's:
Revive: ...no. Just no.
Smite: If you take this, and you're using this guide, I will slap you with a tuna.
Fortify: Somebody else's problem, you really shouldn't care.
Clarity: You may be mana-hungry, but you really don't need this.
Rally: Strangely enough, I barely ever see anybody use this well. Just... don't.
Clairvoyance: Not really use-able if you're following this guide.

Master Yi is underrated. Simple as that. IT IS UP TO YOU TO USE HIM!... or maybe not. Maybe you just wanted to see what he plays like, at that is just fine. There are other guides out there that may go more in depth and more entertaining, but anyway, have fun trying this.

If you think I should add anything, please feel free to tell me.

I'll get around to pictures later...

Thanks for reading :)