This build is here more or less so that I can use it as a reference for a build I'm testing out, however, since I'm posting it here I figure I'll explain my choices for what I'm using right now and why I chose them.

Also, if you want to vote and you're going to vote it down please explain why that way I can take into consideration what you've said and adjust the build accordingly. Voting down the guide before you've completely read through all of it, such as the explanations, won't be tolerated, I can't remove the vote but I will ignore your post altogether.

This guide is not to be used 100% in every game you play. Games are situational and you will need to adjust the build to fit your teams needs and the weaknesses of your enemy.

Build 1
Uses Sorcerer's Shoes and Greater Seal of Health with 21/0/9 masteries for the increased exp gain and health regen with MPen from boots. Great for damage and fast leveling.

Build 2
Uses Ninja Tabi and Greater Seal of Evasion with 21/9/0 masteries which increases armor and magic resist by a little bit while giving 18.75% dodge chance. Great for damage and increased survivability.

Pros
Great damage output.
Very mobile with Shunpo.
Amazing farmer with Killer Instincts -> Bouncing Blades.
Has some nice Phantom Dancers, need I say more?

Cons
Very squishy.
Ultimate is pretty much useless if you get CC'ed while using it.

Greater Mark of Magic Penetration - The usual for casters and AP champions.

Greater Seal of Evasion - I don't have these personally but since Katarina is rather squishy early game these are a good set to use if you grab Ninja Tabi over Sorcerer's Shoes.

Greater Glyph of Ability Power and Greater Quintessence of Ability Power - This way I can have a fairly large damage output early game with my blades so that I can farm faster.

Possible Runes
Greater Mark of Attack Damage - Will work but not recommended over MPen
Greater Seal of Health - Adds early game survivability not really useful late game.
Greater Glyph of Scaling Cooldown Reduction - Works really well late-game in allowing you to spam your skills.
Greater Quintessence of Health - Same as Seals, adds early game survivability.

Recommended
Ghost - Great for running away and escaping, I find it better then Flash since you already have Shunpo however Summoner Skills are purely choice.
Ignite - I love ignite, it's amazing for killing enemy champions who are running away or hiding behind tower and who doesn't love an extra 10 AP when it's on CD.

Possible Summoner Spells
Flash - Works great however you already have a flash like ability Shunpo which is on a very low CD late game, around 5 seconds if you follow this build.
Exhaust - Keeps those runners in range of Shunpo so they can't get away.
I haven't use many other then these so I can't comment on how well they would work with Katarina.

Never Use With Katarina
Clarity - Useless for Katarina, she has no mana.

If you use any summoner spells then Ghost or Ignite adjust your masteries accordingly.

Start off with Amplifying Tome and Health Potion x1.

Try to lane long enough to finish Hextech Revolver.

You should then stay in lane long enough to finish both Sorcerer's Shoes and Will of the Ancients. If you can't finish both then grab a couple Health Potion to give yourself some more laning capabilities.

Grab Rylai's Crystal Scepter for added survivability and it's amazing slow. This will allow you to potentially catch up to runners as long as you can get your Bouncing Blades to reach them through ricocheting.

Rabadon's Deathcap for that major boost in AP. Great item but VERY expensive. If you're having trouble getting this you might want to save it till later and just get the Needlessly Large Rod until you have the extra gold.

Several of you might want to choose something other then Zhonya's Hourglass like Banshee's Veil however I find that on CC heavy teams Banshee's Veil is useless while Zhonya's Hourglass can protect you throughout the duration of the CC and cooldown your skills throughout its duration.

Finally grab a Hextech Gunblade for the lifesteal, active, and AD. Hextech Gunblade is a GREAT item for Katarina because all of her skill aside from Shunpo scale from both AD and AP. This especially goes great with Death Lotus because it causes it to hit quite a bit more. End game without Gunblade your Death Lotus may hit around 200 per blade but with Hextech Gunblade may hit around 240 per blade. I can't remember the exact numbers since it has been several games since I've made it to the end of the build.

If you buy everything in this build you come up to 16830 gold without the Health Potion at the beginning of the game since you purchase Consumables throughout. Since this is a high cost build don't expect to finish it in 3s let alone 5s unless you're fed, farmed like crazy, and the game lasted longer then 45 minutes.

Situational Items
Boots
Ninja Tabi - If you're using Greater Seal of Evasion I recommend using Ninja Tabi and going 21/9/0 in your masteries for the increased Dodge chance.
Mercury's Treads - If the other team is CC heavy and you feel you can sacrifice the MPen grab these.

Item Order
This guide is in no way set in stone, I find these items to be the most beneficial for games all around, however, if you grabbed Ninja Tabi and Greater Seal of Evasion and their team is building tanky it wouldn't hurt to switch out Will of the Ancients for MPen like Void Staff. Also, you do NOT have to buy the items in this order. If you're finding yourself dying more often and unable to hold the lane grab a Giant's Belt before finishing Will of the Ancients and even think about grabbing the second Hextech Revolver which builds into Hextech Gunblade for that increased Spell Vamp. Overall it comes down to your choice, this build will NOT work in all situations.

Now for Skill Sequence, this one wasn't really that well thought out, I decided to test this build in a game but forgot all about the skill sequence, so when I got in I just chose whatever sounded good.

This sequence worked out very nicely because by mid game any creep lower then 3/4 hp would either die or be one auto-attack away from dying from your Preparation to Bouncing Blade combo and it's also nice when you have that runner who's just behind his creeps thinking he's gone and you toss your Blades which go straight back and take out his remaining HP granting you a kill.

Shunpo is maxed second due to the fact that if you can keep this build up it'll be hitting really hard on squishies and enough on tanks to make them think twice about taking you on alone.

Preparation is maxed last since one point early/mid game is enough to keep you going.

Death Lotus, for obvious reasons, is raised whenever possible, this is your damage dealer but it's also a potential death trap for yourself if you don't use it wisely. If the other team has CC such as stun, silence, or anything that can stop a channeling ability make sure that they're all used before you pop in and use Death Lotus, or they're too busy focusing on your teammates to notice you're there.

Farming
Best way to farm is to sit behind minions and let them hit down the enemy minions to about half hp and then use Preparation and Bouncing Blade you'll rack in about 4-5 minion kills each time if done right which grants you a nice amount of gold.

Ganking
If you're ganking one champion you might want to hold back on Death Lotus until the enemy champions run into to save them so if your teammates have nice damage output so just constantly Shunpo and Bouncing Blade the enemy until he's dead.

Teamfights
Against CC Heavy Teams: If the other team is CC Heavy and you can't wait to jump in, use Preparation and Shunpo to the middle wait a second or two for them all to pop their stuns and all on you then immediately go into Death Lotus if done right you should survive, heal all or most of the damage done to you, and have kills or low health stragglers running away. In the latter case immediately Shunpo as close as you can and if Preparation is up follow up with your Preparation and Bouncing Blade. Most of the time this nets me a kill if not all of them.

Early Game (Levels 1 - 6)
Early game just constantly throw out Bouncing Blade on enemy creeps to widdle away at their HP while keeping as much distance as you can from the enemy champions. Use Preparation to Bouncing Blade whenever it is up to hurt the minions that much more. Try to save your Shunpo for last hitting minions when it is safe to do so or for escaping ganks.

Mid Game (Levels 7 - 11)
By this point in the game ganking should more or less be going in full gear. When not ganking or getting ganked yourself use your Preparation Bouncing Blade combo on minions while keeping an eye on the map. Map Awareness is very important in playing Katarina since she can die very, very quickly. If you get an enemy champion low enough to where you think you can take them out and escape use Preparation then Shunpo in, wait for the CC to be cast and then immediately go into Death Lotus if they're still alive throw a Bouncing Blade and Shunpo if necessary. At this point if they still didn't go down I would pull back regardless since their teammates are most likely aware of what's going on. If you're doing the ganking keep to the back of your team and allow your teammates to do some damage and absorb as much of the CC as possible before you do jump in and do your combo. After Death Lotus I would pull back even if you didn't net any kills unless you are absolutely confident you won't get focused.

Late Game (Levels 12 - 18)
At this point in the game you should be half-way or more through your build if you weren't fed and nearly finished if you were fed. From here just run around the map following your most reliable teammate and gank where you can. Use common sense and don't overextend and don't go into fights you can't win. If you do it right and time your skills accordingly your team should be able to net the win easily.

If you finish this build your ulti will hit for 289.94 with each blade so if the enemy champions have no magic resist which you shouldn't count on that's 2899.40 damage output on one champion if you get them alone which heals for 1304.73 with your spell vamp. Now that is what I call an amazing skill.

04/28/2011

Guide Posted
Added Second Build
Deleted First Build
Edited Item Build section to match new build as well as updating runes section.
Added the images for Skills and Items. Also added a Summoner Spells chapter and a Game Phase chapter, added total Item Cost in the Item section and a new Chapter.

04/29/2011

Added mini-situational items for boots, plan on adding more as I test this build out in more diverse games.
Added second build with different boots and seals.