Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
My Katarina build is fairly standard in the beginning but tends to work best in team builds with a jungler so she can solo a lane but still have backup if needed. Do general harass level 1 - 5 (don't be over zealous going for kills) and once 6 I usually attempt to go for an ulti gank because the solo lane will have me 1 - 2 levels higher than the opponents. Other than that just buy sorc boots asap and try not to B until you have enough gold for at least a mejai's. If possible try to save up for both mejai's and leviathan's and purchase at the same time. This is probably the most risky part of the game because if you don't get your stacks up you are going to have a really hard time.
All in all I love this build because if your gameplay is solid and you manage to snag a few quick ganks after your mejai/leviathan combo you tend to just steam roll for the rest of the game. This early combo allows for pretty decent survivability and excellent damage output in the late game as long as you keep alive. Over all this is my favorite way to play Kat.
If you have any suggestions feel free to let me know :D Thanks in advance for your opinions. Oh and please note my summoner level is only currently 21 so I'm sure final changes will be made once I reach 30 and fully realize what runes etc are actually most beneficial for my style. I look forward to reading comments and suggestions so thank you in advance for your help.