Hello, my name is Extract im a Master Katarina player and i will give a you a quick guide to reach your desired rank this season ^^

This section will be put in shape over time so be gentle with me, for now on this section are mostly my thoughts and experience with Katarina

Combos are important, dont **** them up.

Standart all in is E->W->Q->Gunblade->R->E

I will add some Videos later ^^

The most important rules are:
1. Be quick
2. Be clean
3. Dont go first in teamfights
4. kill the carry
5. use your Powerspike
6. Farm

-One of the most important points is to calculate the "risk reward" outcome of a all in or roam. As an example is it worth to roam botlane and double kill the Jinx and Nami?
If you get both kills dont die and the other midlaner is not roaming with you or pushing your tower its a completly worth, but if you roam without pushing out the wave and die to jinx without a return kill you will fall behind eventually and be useless later on. Sometimes its more worth to stay in your lane and just follow the enemy midlaner when he roams, because not like the most other assassains Katarina has a relativly great teamfight and scaling! BUT keep in mind that a Katarina is one of the most snowbally chmapions in the game. If your botlane gets pushed in and you have the slightliest possibility to roam down go for it and because you are Katarina you dont even need to hard focus the enemy adc you can just kill the squishy sup and then the adc afterwards. (keep in mind that ou dont jump onto a Nautilus if you are 0/0 without a Gunblade or magic penetration) early fights pre 6, after 6 and pre midgame are favorable for katarina because of her resets. A good roam will give your Botlane air to breath, reset, farm, dragon or get first turret AND it will bring the enemy botlane behind.

-Dont be shy to ask for summoners or ping them yourself. Most midlaner or adc without flash are a easy target for Katarina

-keep in mind that you the only champion that gets his whole kit resetted after a kill or assist. In a teamfight where your team gets a good engage or gets engaged you nned to position yourself out of vision from enemies or inyou abckline and wait for a good oportunity to go in.(get sweeper as fast as possible to be invisible when roaming)If the enemy tank is engaging ur adc or backline its your time to come in play. Track cooldowns!!! Veigar cage is out and Nami just used her bubble, time to shred their backline and with darkharvest you can pop all of them with just e resets or Gunblade. You dont need to channel your ultimate completle through if a enemy is on 20-30% hp you can e to him and kill him with e -> aa -> q and jump onto the nect target.

-learn to bait your daggers, if you throw down your w and run a way from an enemy you can castyour e on max range to a dagger and get on the opposite site of it to trvel extra distance.

-Katarian get resets of her passive on clones as LeBlanc clone, Shaco clone and maybe Illaoi and Neeko soul and clone,but im not sure with those 2 (didnt play against them in ages)

-all in on Cassios is easy when you e to her and do your whole combo with R when facing away from her before channeling your R, because Katarinas ultimate doesnt make her spin, she is standing still in reality so all Cassios will R you instantly and be confused when youre not stunned.

-you can e over thin walls when throwing down her W and instantly E on the other side of the wall. You can also E on playnt, blast cones, vision cones, minions and allied champions.