*NOTE*

This guide/build is 99% finished, i just need a few comments, a few votes and some advice to further develop this build into perfection!

First of all, I admire your courage to even begin with this guide. :P
This is my first guide ever, but I have played many games with Corki, so experience beats quantity. You may notice that I have added much items, but this is meant to let you know when you need to go back, if you listen to what I say you to do, or else you will need to go back more often because of near deaths. The most important thing for Corki is:

1. Solo lane (if possible, if you choose not to get a solo lane, get a lane with someone you know (very) well or understands that you need to farm.)
2. Farm
3. Farm
4. Farm
5. Stay in your lane as long as possible! This allows you to get more exp and money from minions.
6. Make sure you get that money, you need it for items so you can PWN!

The runes I provided here, are imo the best to use.

The Greater Mark of Desolation is a good choice for the armor penetration, as you are an AD champion.

The Greater Seal of Clarity is the best choice for Corki, as he uses his mana like a tank does when he starts; the mana is there at the beginning, aaaaaand it's gone when you use 3-5 abilities. So make sure you get these!

The Greater Glyph of Warding is a good choice, though some people are used to get the Greater Glyph of Shielding, because in the end, you have more MR. I have noticed that most of the games I play with Corki, end before I am actually lvl 18. Also, early ganks and other AP champs can get the better of you when you don't have enough MR. Like I noticed just today 2/20/2012. I got an Ashe against me, and she did massive damage (rune page without Greater Glyph of Warding). Just make sure you have some MR early game, and because Null-Magic Mantle isn't one of the beginning items you take as Corki, you will need MR from either Masteries or Runes. As a Corki, you'd do good not to take MR from masteries..

The Greater Quintessence of Strength needs no deep explaination. Corki needs his AD, and he needs lots of it! Just the best Quintessence for Corki, no doubt.

I went for 0/9/21 because as Corki, you need lots of mana&health and lots of mana&health regen. It also is kinda usefull to select Ghost and Flash when you are going to work with this build, as you will see that, as a good Corki player, you will need these for just that last, perfect Rocket Barrage(s). Also, ability cooldown reduction is also very welcome as Corki, as you will need the extra reduction for, as I said before, those last, perfect shots and Rocket Barrage(s). You will see that Rocket Barrage is one of the best finishing moves, as it takes little mana to use, and has an uberlong range, does reasonable damage with normal shots, and big damage at the fourth missile.

EDIT 2/20/2012: As one of the patches gave League of Legends new mastery pages, I must renew this too. I went for 21/2/7. As I am playing Corki alot, I noticed that, when I used my old masteries, I'd just not have that extra AD that kills. I went for a new mastery selection, that DOES have this extra AD. I still kept the mana regen, as you really need this while playing Corki. That little bit of extra armor will come in handy too, at the beginning of the game.

Why AP?

Well, Corki needs it. And it is always usefull to have, though most of his damage comes from AD. And, I needed it to get 20 points into the Offense tree :P

No Deadliness? No Lethality?

Nope. Corki won't need this, because the abilities and the basic attacks of Corki will do enough damage to take down an enemy. Or more. I have invested these 5 points into the Utility tree, so Corki gets extra mana and mana regen. I favor mana and mana regen above a little bit extra AD an CSD.

*NOTE*

I will say >can< at every buy, because I recommend taking this item, but it is not required. You can also follow another buy order, but I myself prefer this order because it matches my gameplay!

*ANOTHER NOTE*

RED BUFF = CORKI'S BUFF, REMEMBER THAT

As you people can see, I have set out a buy sequence. I have added every item you need to buy when you return to base, if all goes as planned.
So, at first you can buy Boots of Speed and three health potions, as this will come in handy when you want to stay in your lane as long as possible.

Important! When you reach 400 gold after purchasing Boots of Speed and 3 health potions, return to the base immediatly for the next item, Mana Crystal.

When you are playing for about 10 minutes or so *without* Mana Crystal, you will notice the fact that you are out of mana, further named as oom, alot. So, what you want to (can) buy next is a Mana Crystal, so you won't be oom so many times and be able to keep harrassing the enemy. After a while, you will notice you can stay in your lane even when low on mana. You can do pretty much damage with your auto attack, and harrass the enemy easily. In time, you will need mana regen, as you want to kill your enemy/enemies, and spamming Q will take mana quickly, when doing so you will see that you are going to be oom often when wanting to kill your enemy you just lowed. I'd recommend to further your Mana Crystal into Tear of the Goddess, as this will give you good mana regen. And as you will be harrassing your enemy/enemies and spamming your abilities once you can, your mana will greatly increase,

BUT

, you will not have Tear of the Goddess long enough to really get 1000 extra mana from it's passive. When you are going back for Tear of the Goddess, which will cost 595 gold, make sure you can also buy Long Sword, for that extra damage. So get around 1050 when you return for Tear of the Goddes and Long Sword. After those buys, you will be able to stay in your lane longer than anyone. You can now buy Manamune first, to get your first end item. When you have enough gold to buy Manamune, I strongly recommend doing so, because you will need to get as much mana as possible from it's passive.

BUT

you can also choose to stay in your lane a bit longer, and can get Dagger or Vampiric Scepter for further item development.
The stage where you are now is the 'gank stage' where you, how surprisingly, are going to gank as many enemies as possible with your team. I assume you are midlane, else you don't need to follow this advice, and skip the next few lines, and thus easily 'gankable'. Make sure your allies cover you, place wards and make sure you and your allies

CALL SS OR MIA

. I prefer SS, as this is used more often, and most players understand this. When I am assuming right, you will now have:
1. Boots of Speed
2. Manamune
3. Vampiric Scepter
4. Dagger
5. Health Potions (this is not common, as you should've already used these)
Ganking is one of the things that make Corki a hated enemy and a loving ally, as you should've already made some kills in your own lane, and thus are stronger than normal. Make sure you advice your teammates of ganking their lane enemies. Get in the nearest bush, ready to ambush them.

*BEWARE*

Ask your allies if they saw an enemy placing wards. If so, take another route, or even wait with ganking. When you are ready to gank the enemy/enemies, think about your placing.

Option 1:

When close to the enemy/enemies, you are going to start with Q, immediatly followed by pressing E, auto-attacking, and than firing a few missiles with R, until you have hit your enemy/enemies with the big Rockter, but this is not common, as Corki will do much damage with:
1. Q, Phosphorus Bomb
2. Auto Attack
3. E, Gatling Gun, this is really useful when chasing your enemy/enemies just to apply that extra damage.
4. R, the normal rockets
and thus Corki will kill fast when working together with his teammates.

Option 2:

When the enemy/enemies is/are far away, you start by jumping in with: W, Valkyrie into the direction your enemy/enemies will most likely flee. This gives you an extra element of surprise, because they would never suspect you would cover such distance in such short time. (Same is done with Fiddlesticks, who can, with his ulti, cover a reasonable distance with the 'teleport' of his ulti, most players don't expect him to do that when he is about to gank when you don't know it.)
Then, do the same as option 1, just spam Q, Phosphorus Bomb, and continue to shoot the enemy with R, Rocket Barrage until you have a kill, maybe even more!

Next, at this point, which one you take is your own choice, but if you are low on health most of the time, which is not really common for a Corki player with this build, you can do good taking The Bloodthirster first, and after that you can immediatly take The Black Cleaver. You need those two items for sure, as they are most important to Corki, because he needs massive Attack Damage.
You will now be around 30 mins into the game, maybe even 40, and so that means you are entering the 'turret stage'. This means that you are going to destroy the enemy turrets asap. This will be able when:
1. You have some kills
2. Maybe even fed
3. Your team is in the same lane as you, as you carry the team. (Backdoorers not included! ;p)

Ok, so now we, or well, you, are in the 'end stage', this means that one team is going to win within the next 10-20 mins, depends on the actual ingame time. You can now buy either Aegis of the Legion OR Frozen Mallet. This choice is easily made by asking yourself this question:
Does anyone else in your team have Aegis of the Legion?
If Yes, take Phage first, after that, further it into Frozen Mallet.
If No, take Cloth Armor and start building that into Aegis of the Legion.
Next you can buy Berserker's Greaves, if you haven't already done that, which will be rare. When you have just one more item storage thing empty, you can buy another Mana Crystal and further that into Sheen, and you evolve Sheen into Trinity Force to apply the last finishing touch to complete this godly Corki build. And thus, as you can see, you will need loads of money to complete this build, and you will often not have the chance of getting this build complete. Make sure you jungle now and than and kill as much minions as possible!

You will, 80% of the time, win the game when you are fed on Corki,

OR

you have a monstrous farm on minions.

The sequence showed in the Ability Sequence on top, is the most common sequence I use, but sometimes this is not the best order to follow, as you sometimes need Gatling Gun maxed out earlier that you need Valkyrie maxed out, and sometimes it's reversed.

I'd recommend taking Q, Phosphorus Bomb, first, as this is very useful and does great damage on low level. I'd choose E, Gatling Gun, as second, and after that W, Valkyrie, so you have 1/1/1/0. After that max out Q, Phosphorus Bomb, first, and combine E, Gatling Gun and W, Valkyrie together to get a sort of general skill selection. I don't have really much to add, just that if you are being chased all the time, go for Valkyrie first. If you lack minion farm, go for Gatling Gun first.

Well, just because the fact that as a Corki who is fed or has a huge minion farm, you will be the #1 target, and if you are being ganked while fed, you need to get the hell outta dodge! This is the reason I went for Flash, as this will get you out of a nasty situation with the blink of an eye ;) Ignite is a good choice, as most of the times you will just not kill the enemie. Having them 'Ignited' will be the final decision about their lives. Make sure you get this. =]

Only one more thing to say:
PLAY SAFE when solo laning!

Don't chase enemies when you are below 25% health, and there are no allies near you.
Don't be greedy. You

are

allowed to ks! ;p

And most important of all:
~ have fun!
~ good luck!
~ and play fair!

I have built this build/guide all by myself and my own experience with Corki is weaved into this build/guide.

Post pics or stats when you have tried out this build with your comment pl0x!

This build/guide is brought to you by me, GalowHeLL!
My LoL name is: 'ChurchillMk2' (exlude the '') may you ever feel the need to practice with Corki, I'll be your instructor!