Perturbed Python

I've been on eBay recently and I saw some really cool minifigures, something that really stood out to me was the helmets (I'm talking starwars here :P) I really want to make my own mold and make my own well.. helmets. What do I need to accomplish this task? Is there a cheap alternative? I bought "Instant Mold" and I'm waiting for it to arrive, but I think the main problem is the cast. Any help is appreciated.