Items: Pretty much whatever. Tear of the goddess asap, start out with Doran's Ring and pots. Archangel's staff isn't bad, or you could go for a quick rod of the ages instead. Just remember, AP and mana.

Early Game Laning Technique:
QQQQQQQQQQQQQQQQQQQQ [hit level 2] WQQQQQQQQQQQQQQQQQWQQQQQQQQQQQQQQQQQQQQQQQWQQQQQQQ QQQQQQQQQQQQQQW [hit level 6] R

Mid Game Laning Technique:
QQQQQQQQQQWQQQQQQQQQQQQQQWQQQQQQQQQQQQQQWQQQQQQQQQ WQQQQQQQQQ
remember to use "R" if you need it

Mid/Late Game Ganking Technique:
EWQQQQQQQQQQQQQQQQQQQQQQQQQQQQQWQQQQQQQQQQQQQQQQQQ QQQE [if they're not dead yet] R

And, yeah. That should do it.

Filler [what to do during all other situations]
QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQq

What to do if you're losing:
QQ

Fantastic guide Lightstriker! Without a doubt one of the BEST guides available. If somehow you are having trouble mastering Karthus gameplay, this guide should really help you out.

A+

The only other thing i would note is that stacking cooldown reduction makes Q happen fasterand therefore is crucial to Qing as often as possible.
Otherwise this guide really seems to capture the essence of Karthus, nicely done!

This almost had me in tears.

Dude, this is an evelynn guide, not a Karthus guide! DUMBASS!

This is win.

Oh i see what you did there,

excellent work