Hello I'm Apo and I am a katarina player for over 4 and a half years. My peak was D1 and I have over 2 million mastery points on Katarina. Katarina is an extremely agile and bursty champion, with a high-risk high reward playstyle. She is super rewarding when played right and her kit is fully understood.

Pros – Is very bursty, Is a very mobile champion, Can easily get picks at any side of the map and goes very well with morgana.
Cons – She's very squishy and can get cut off very fast by CC. Most of her matchups are opponent sided.

Compositions good with Kat:
Katarina goes really well with champions who have heavy CC or are very tanky. Some examples are Ornn and Morgana

Comps bad with Kat:
High bursty comps make it extremely hard to get fed as everyone is competing for kills.

Synergies to help you win your lane are : , Elise , Volibear , Gragas , because you can easily proc your daggers and get a free kill.

Dark Harvest - low cc, one or no tanks, scaling for late game.
Conqueror - high cc, lots of tanks two or more, hard early to mid game match up, high poke

Runes aren't really interchangeable other then the ones above, other then the scaling health rune to the depending match up damage type.

Always go Flash and Ignite since it provides everything you've ever needed as Katarina. Never change them for anything else except Teleport when you're on the top lane.

I'd currently suggest going the AD build because it's just so strong and you have nothing to lose by going with it. Most of her abilities scale with AD and AP so your first item of choice has to be Hextech Gunblade. Other than that i'd say follow my AD Kat guide since its so strong currently at 10.2

Katarina's kit is very simple but comboing on her is very difficult, especially in high ELO.

Passive: Voracity
First Passive: Innate: Whenever an enemy champion dies within 3 seconds of Katarina damaging them, her abilities' Cooldown reduction icon cooldowns are reduced by 15 seconds.
Secondary Passive: Sinister Steel - Whenever Katarina retrieves a Dagger, she uses it to slash through all nearby enemies, dealing 68 − 240 (based on level) (+ 100% bonus AD) (+ 55 / 70 / 85 / 100% (based on level) AP) Magic damage magic damage and reducing Shunpo Shunpo's Cooldown reduction icon cooldown by 78 / 84 / 90 / 96% (based on level).
Daggers last 4 seconds before disappearing.

Q - Bouncing Blade - ACTIVE: Katarina throws a Dagger to the target enemy that bounces to up to 2 nearby enemies, dealing magic damage to each target.
MAGIC DAMAGE:
75 / 105 / 135 / 165 / 195 (+ 30% AP)
The Dagger then lands on the ground 350 units opposite from where it struck the target. The time to land from striking the target is the same regardless of distance traveled.

W - Preparation - ACTIVE: Katarina tosses a Dagger into the air above her current location, gaining a decaying bonus movement speed until it lands after 1.25 seconds

E - Shunpo - ACTIVE: After a short delay, Katarina Flash blinks to the target location around the target unit or Dagger, dealing magic damage to the nearest enemy in range.
MAGIC DAMAGE:
15 / 30 / 45 / 60 / 75 (+ 50% AD) (+ 25% AP)
Shunpo resets Katarina's basic attack timer.
Upon blinking, Katarina remains in Channeling for 0.15 seconds. Movement command buffering is also delayed by 0.85 seconds.

R - Death Lotus - ACTIVE: Katarina rapidly Channeling for 2.5 seconds, throwing a dagger every 0.166 seconds to up to 3 nearby enemy champions, dealing magic damage and applying Grievous Wounds for 3 seconds, reducing healing on the target by 60%.
MAGIC DAMAGE PER DAGGER:»
25 / 37.5 / 50 (+ 22% bonus AD) (+ 19% AP)
Katarina does not need to have Sight icon vision of the targets to hit them, and she will prioritize the closest ones.

In the early game, Katarina can get off some sweet damage very easily by using q on a minion and jumping on her opponent with e and dagger proc to get some poke and electrocute proc(if youre electrocute). Try to harass your opponent as much as you can.

Mid game is where Katarina starts so shine. If you have atleast 1-2 kills and have a gunblade in your inventory its your time to control the entire rift. Start roaming bot lane(most of the time) and do jungle invades to try and get some free kills since youre one of the strongest 1v1 champions.

At this part of the game Katarina is probably the strongest one in the entire rift. Use your lead to get picks or even all in with your engage if you have zhonyas and use your Shunpo to get back out. It should be an easy win if you have a half decent team.

Add: Apo#0420 on discord if you have specific questions.