This is my first guide and mainly making it cause I'm bored, anyways.

This guide is very effective in both normal and ranked games.
I strongly recommend you to focus on CS on your own lane atleast until you have your RoA and boots, after that some roaming fits perfect, by then you're pretty beefy + have high dmg. And as always try to pick up free gold with ult, don't only use it to KS, use it to initiate aswell.
Late game you will probably die alot which shouldn't be that bad, you should always try to locate to the middle of the teamfight and just spam your Q while E is active ofc and don't ult until you're dead or they're not able to interrupt it cause it's quite easy to really screw over Karthus by silence/stun/etc...
You will get some serious teamfight sustain when you get your Hourglas aswell.
Karthus win MANY teamfights for his team by just being there with his E activated so if you miss a few Q it's not that big of a deal even tho the add great dmg.

(Not doing any spell check at this)