Katarina In-Depth Guide

Reposted from an old comment of mine for the sake of sharing knowledge:

I'm not the best Katarina player, but here are some tips that might help (for you mains out there, correct me if I'm wrong, please)

Laning/Trading

General: Usually Katarina's job in lane is to out-sustain her oppponent. When I say out-sustain I don't necessarily mean in terms of your health vs. their health, but rather your health vs their mana (if against mana-using mid lanes of course). An enemy laner with no mana is as good as dead, and Katarina's job is to jump on this opportunity.

Creep-score/last-hitting: Since Katarina is melee, she can have trouble last-hitting against most mid laners. The main thing to remember is that it's okay to miss CS, and that you shouldn't be taking free damage for one or two minion kills. With patience, you will regain the CS you have missed from zoning with proper game knowledge and wave control (freezing the wave in good spots whenever you can, and learning to farm minions under the turret) which can be accomplished through general gameplay. Also, learning what you can do while zoned out, which blends into the next category; trading.

Trading: A big part of Katarina's kit for laning (in my opinion) is the effect of her Shunpo (E). After using it, she takes 15% reduced damage for 1.5 seconds. Because of how it is stated, it reduces ALL damage taken including true damage. However, I wouldn't go wasting your only mobility on a minion/monster/enemy two inches from you just to keep off minimal amounts of damage unless it is completely vital. Instead, this ability makes Katarina exceptional with trading, mixed with her Sinister Steel (W). Once the enemy laner has used a core ability and you have either dodged it or plan to dodge it, you can Bouncing Blade (Q), then Shunpo (E) and Sinister Steel(W) to get on top of the laner, proccing the Bouncing Blade (Q) mark, proccing your Shunpo (E) damage reduction (mitigating any return fire) then causing damage and granting you movement speed from the Sinister Steel (W) to reposition. (Sinister steel only grants movement speed when it hits enemy champions). However, this doesn't include regards to jungle/roam pressure, so be careful when choosing to use your Shunpo (E) as you may get ganked afterward leaving you with no mobility left to get away. (If you know you can kill the target, go all for it, since your Shunpo (E) will reset from the kill)

Roaming: Roaming is an extremely strong strategy during the laning phases of the game. This is how you can "carry" games and recover a losing lane (or even start snowballing for a lane advantage of your own). Good times to roam are when you have shoved lane, are coming back from fountain, or objectives are being fought over. Roaming is an easy-to-do strategy but is not abused like it could be because people seem to think CS/levels is king (they are important but in my opinion the simple chance to help teammates or create a snowball for yourself is much more vital). Before you decide to roam, keep tabs on the junglers and decide if it is a good idea to go where you planned. For example, if you are ahead and you notice that your bot lane is overextended and might get ganked, your laner has gone missing, or a lane could just use some help push your lane (if possible) and roam to the bottom lane to prep a counter-gank (that's right, it's not only the jungler's job to do this. Also, keep in mind about enemy top laners with teleport that could be a nuisance). This can set up easy kills and great objective control to build to an early victory. If you are behind but the enemy jungler is top lane, heading for bot to fight a 3 v 2 is an excellent idea as it can get kills and completely secure objectives like dragon or the enemy buffs.

Teamfights

This is where I'm a little rusty. Katarina's team-fighting can be very difficult or extremely easy depending on the enemy's composition/your team's composition/situation/who's fed/who's not fed but to put it blatantly, patience is the answer. Notice who's on the enemy team, and keep track of major CC abilities that could ruin you. Once you are certain you can either burst down someone in the blink of an eye (while being able to return to safety), or sure that you can output large amounts of AOE damage without getting killed/stopped, then you can use that blinky-blink to hop on in and scoop some cash. Also, Zhonya's Hourglass ( a must-have on Kat) can make team-fighting much easier, but CC can still keep you from casting it, so be wary of that.

Build/Items

(Start boots and 4 health pots for lane sustain -- Health over Mana in lane) Katarina's build is that of any melee mage/AP assassin such as Diana or Akali where you want to rush either Abyssal Scepter (if against AP) or Zhonya's Hourglass (if against AD) but can change depending on the situation. After you build either Zhonya's or Abyssal, build the other one next unless a team is AD heavy/AP heavy which in that case Rylai's Crystal Scepter is a great utility item to build second or third. If you get fed and don't think Zhonya's is needed (I almost always take Zhonya's over Rabadons first because of insane playmaking/initiatin) then Rabadon's Deathcap is the next best choice. Followed by a Void Staff and then your choice of either Guardian Angel/Banshee's veil or an offensive item. (General full-build example -- / states that order is situational--: Abyssal/Zhonya's, Sorc Shoes, Rabadon's/Void Staff, Rylai's Crystal Scepter/Situational Item)