This is a really horrible guide made by an amateur. Don't bother reading this unless you're that bored.

I took attack damage reds because she scales off of ad. You can also take magic penetration reds;I wouldn't because she would already have enough magic pen. from this build. I took armor yellows because she is most likely going to get harassed for being a melee in mid. I took ability powers blue and quintessence because it compliments her damage. You can take ability power scaling runes if you prefer.

Go crazy with the masteries.

For the "How I Build" section, I would usually rush Deathfire Grasp into Sorcerer's Shoes . I chose Deathfire Grasp because it gives you a potential to snowball; It increases your damage and helps with getting resets on your passive. It benefits Katarina because she can play an assassin role. I would choose Sorcerer's Shoes because of roaming potential. After I finish these two items, I would get Rabadon's Deathcap for the bonus damage and Void Staff for the extra magic penetration. The rest of the build is up to you. I would build Zhonya's Hourglass and one of the optional items listed.

Standard Skill Sequence
Against's Champions Like Yasuo
I would max w against people like Yasuo because it's just better in the long term. It's nearly impossible for him to escape you with your constant w and movement speed buff. It would be extremely hard for him to kite you with his e.

Why not take e instead of w? I wouldn't max e in any situation. E is more or less like 1 use against people like Yasuo. W has a low cooldown.

The summoner spells I would take for Katarina is Flash , Cleanse , Ignite , and Barrier . I would always take Flash on Katarina;it can be used to engage onto the enemy carries or to just escape. Cleanse would be a good summoner spells as well. It removes all debuffs and allows Katarina to continue her role in teamfights. Ignite is also a good spell for Katarina. It applies damage and can help you get resets. Barrier would be a useful skill to have against high nuking champs like LeBlanc.

1. Ignite Flash
2. Barrier Flash
3. Cleanse Flash

Playing Katarina:
Katarina is one of my favorite mid laners. She is easily countered though; she can also counter most mid laners that rely on skill shots like Lux or Ziggs. When a teamfight erupts, I would wait to go in. Your best chance to go into is team fight is when the enemy team is low and they blow all of their displacement or stunning abilities. Keep in mind that Katarina has no crowd control for herself and is generally squsihy.

Playing against Katarina:
Katarina is a champion that can snowball. When she is missing call an m.i.a. or follow where she is going. She can snowball heavily off of kills; she can also snowball in teamfights even if she is far behind. In teamfights you want to save an ability that can stop her from ulting. When laning past level 6, save your stun or displacement ability for Katarina's Death Lotus. Her #1 enemy is anything that stops her ult. When laning against a Kat, feel free to push her under tower because she is not a strong farmer under tower with items. A good counter for Katarina is Malzahar Diana and Lissandra. All of these champions have the ability to stop her from ulting.

I would not recommend her in high elo. People are smart enough to focus Katarina first in every teamfight.

During laning phase it is safe to farm with your q. Her q can offer harass and farm at the same time. I would not use e to get any farm because it leaves you vulnerable to ganks and skill shots. A way to wave clear is Bouncing Blades the minion wave and then sinister steel to proc all of the marks. Don't be afraid to spam your abilities to farm because you don't use mana.

Pros
1.She has decent wave clear.
2.Her passive Voracity gives her the ability to clean up.
3.She can counter most skill shot reliant champions like Lux.

Cons
1.She gets countered easily.
2.She has no crowd control.
3.She can be easily pushed out.

Katarina is great with a team that has aoe and cc. The aoe compliments her ult and the cc makes up for the lack of cc Katarina has. The aoe compliments her ability to get resets after resets.