Hi! This is my first build, featuring Katarina Jungle, a favourite of mine. Definitely try this in normals, preff. with a friend or two before trying it in ranked. I will go over the abilitys, runes, masteries and build I run with Katarina jungle. :)

Pros:
Very damage heavy late game
Rare, so people don't really know what to do against you
Cool as ****
Good clearing after a few items
Good burst ganker after 6

Cons:
No CC
No Sustain early game
Weak early game clear
Not great ganks pre 6

I prefer to max Sinister Steel first as it is a spammable and high base damage AoE attack, making Katarina's bad early game clear a lot better than what she would normally be. However, maxing Bouncing Blades may as work as it is also AoE, and procs an effect on the next auto-attack on affected targets.
Of course, put a point in Death Lotus whenever possible as it is an amazingly cheap ability that can clear an enemy with such speed it's truly astounding.

You will need to communicate with your laners before going in for a gank, as Katarina has no cc, meaning that your allies will need to either bait the enemy into attack them or use their own cc to allow you to come in and do damage and minimise their chances of escape. This means you must be pinging the lanes as you go to gank them, telling them when you're ready to attack, etc etc. Voice chat works very well for this (Skype, Curse Voice, TS, etc etc again).

Katarina in any role has great team synergy in team fights as she can combo the ADC and then use her ult to finish the ADC off, while also dealing tons of damage to all nearby people.
People who synergist well with Katarina are as follows:
Lissanda, Orianna, Nunu, Miss Fortune, Leona, Jax, Jarvan IV, Syndra.

That's it for the guide, thank you for checking it out, if you do play Katarina Jungle, hope you enjoy it as much as I do and good luck on the Rift, Summoner. :D
Thank you <3