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.
(Sorry, my english is bad i know, but i'll try to share my build and make everything easy to understand for you :P )
Ok! I'll make it quick. This is MY way to play Katarina on the middle line. First of all, Shunpo is your best friend, upgrade it whenever you can and don't forget to use killer instinct before using shunpo!
Well, at the start of the game, buy the doran shield + 1 health potion and head for middle line. AS SOON as you can see your enemy jump on it with your shunpo spell. Due to your high amount of health (Your defense masteries + the doran shield) you should be able to handle almost any guy who's in front of you. Then keep attacking him and use your potion during the battle. ALmost all the time, people will try to fight you, cause they want to see you dead. No matters what happen here, use your Ignite spell on it and then heal yourself with your heal spell if needed while continuing attacking him with physical attack. Now, you should kill him and if not, just shunpo one last time on it and he's dead. It work versus almost every character who don't have more than 4 life bar, and even vs 5 life bar. If this is not working, just wait until lvl 2 to get your bouncing blade and harass them with it, then shunpo to kill them.
Depending on how you,re good, you should be able to get your belt pretty soon, granting you more hp. Now, the fun is starting. Don't be afraid to go kill people near their turret, you got enough hp for it now, they can't run from you. It's now the time to gank, get the rylai staff and all the other good item for Katarina. For myself, i get the rylai staff before the Mejai for one reason: Katarina is killer in mid game and you'll get your rylai staff at the start of the "mid game" session + it'll be more easier for you to get your stack with this wonderful item that slow your enemy while using your death lotus.
With 14 stack and more, all the item listed in this build and a some good skills, you should be able to handle the end-game really easily but don't forget to stay in team if you don't want to loose your stack or being gank by 3 to 4 guy at the same time because the other team will consider you as a real nemesis now.
Well, hope you like my way to play Katarina. If you have any question, comments, you're free to share them!
Good game ;)
(Sorry again for the bad english :P )