You'll need to hunt for your pumpkin in a place with weird-shaped pumpkins, which (as I mention in my Life-Size Skeleton instructable) probably isn't your local supermarket...
You need a pear shaped, head-sized gourd. It can be shaped like a pear or a lightbulb, as long as it's got a nice round chunk where one might keep one's brains, and a narrower area that might serve as a jawbone. Often a pear-shaped pumpkin will be a bit skewed to one side: these are especially good for our sinister purposes.
Pick up the candidates. Recite some Shakespearian Yorick-related verses. Think "skull." If at some point this process gives you either the willies or the creeps, then you've probably found the right pumpkin.
Take it home and rub your hands together, cackling, and bust out your permanent marker.
Draw out your shapes, using photos, toy skulls, or actual skulls if you have them laying around.