You have to buy these items in this order in which i wrote them. I always play Karthus at mid. You have to start with the q- skill and just kill minions until you get your ulti. Then you watch out on the lanes and kill the enemy champs with your ulti to get fed. You wont have any mana problems with that build. Have a lot of fun with it,
greetings,
Tobias

~ i am not really sure, if the order of the skill is possible..i can not recognize :-D but you first have to max q, while you max the ulti (r). Then you have to max the wall ( w) and then the aura(e)