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.
Item build is made for Katarina to explode her AD power. I usually start with Doran's Shield, to get some life on early game, so its more easy to hunt your enemies if you take de mid-lane.
-The heal Potion can be used if you play offensive. Otherwise, you might not buy it and save some money. I try to play off so I can easily get first blood because the enemies at mid-lane are sometimes squishy heroes
-Sorcerer's shoes / Boots of Mobility. I always choose the first one, because my blades are more efficient with magic penetration since these have magic damage.
-Rylai's scepter. I usually take this item, so my enemies have no scape since my rune build is made for cd's decrease and I get more life because at late game Doran's shield is useless
-Finally The bloodthirster. I take this items because for me its the more efficient when you want to make a full AD hero. Katarina doesn't need critical damage so Infinity Edge is useless too, a complete BT gets +100 Damage, against +80 from IE. Completing your build with 4 BT you would get +400 damage, so your Death Lotus will get more than +500 attack power per blade
My Skill Build is made for a full offensive play, since Killer Insticts just add a bonus magic. Thats why I level it from lvl 17. I gave a level at lvl 7 to get a magic bonus because at that level I wont have much AD. But then, for me, it's useless.
Mastery Tree , as you can see, is FULL offensive too. In early game i will need more critical damage, because I wont make much more damage that the one I have from the beginning.
Try it, and good luck mates!