How do you level up on black ops 2 zombies?

The ranking up system consists of kills, revives, perks drank, headshots, etc. The levels are : 1 Bone, Crossbones, Skull, Skull & blue eyes, Skull & 2 Shotguns, & Skull 2 Shotguns, & blue eyes. In that order. There is also tally marks. That shows how close you are to ranking up. Lastly, it also is the rounds you make it up to. I'm max rank. Hope this helps!