This is the first time I do a build on this site. I used a lot of builds to finally create this one. I'll try to make it quick so you won't fall asleep reading it. Here we go!

First of all, BOOTS OF SPEED and HEALTH POTION for the first minute gank. You EXHAUST and IGNITE for some more damage and you try to SHUNPO in and out if necessary. Usually, I try to take top when the minions spawns cause you get to max out Bouncing Blades real quick (this will be your first source of income and will get you the very "last chance kill on the guy who's running away from me" most of the time). When you have enough money, you buy the NEEDLESSLY LARGE ROD for your Boucing Blades to deal more damage (and Shunpo at the same time). In fact, you try to be not too agressive (do not chase, you'll end up being ganked most of the time cause Katarina is a natural target). Kill minions, harass the enemy champ a little bit and play safe. If you're not going to kill, DO NOT FEED, that's what I say. In the first 6 to 7 minutes of the game, you do that, you earn money and you buy the items.

Since you probably have a decent skill tree by now (and this means you have your ULTIMATE), you can start to finish off the enemy when your teammate cannot (this mean DO NOT intentionnaly KS). Still play safe, don't try anything you are not 100% sure to finish. You'll earn money for killing Champs and minions will just crumble in front of you like a cookie left in the couch's crack. At this point of the game, you should at least have the NEEDLESSLY LARGE ROD and the IONIAN BOOTS OF LUCIDITY. If you already have the RABADON'S DEATHCAP it's even better.

Now you should be able to solo a champ (if it's not a tank) in a 1 on 1 kinda easily (with EXHAUST and IGNITE) and you should also have the HEXTECH GUNBLADE wich will help you cause of the lifesteal. FROZEN MALLET is not THAT important to have cause seriously you'll do so much DPS that the enemy will either flee, or die. From there well, you kill (minions or champs doesn't matter unless you KILL) and it's the best time to play with your team cause when the fight will go 3 vs 3 you'll jump in the fight ULTING the enemy ***es. Usually, at 22 or 25 minutes, if you handle this build correctly, the other team will surrender (or try to). Hell, for the last 5 games I never destroyed a Nexus...

This might not be a perfect build, but this is what fits me the most. You can decide to go with B.F.s if you wish, you will still rock, but if you go B.F.'s build try it with the RABADON'S DEATHCAP. You will not regret it. Good luck and remember, play carefully, do not chase too far and don't do things you cannot finish. If you can't kill, DO NOT FEED. Oh and leave comments!