Alginate

Alginate is the material used to create the impression of the skin. It's specially designed to be safe for use in contact with the skin, unlike typical prop-casting materials.

Alginate is a dry powdery product derived from seaweed. When mixed with water it forms a thick viscous liquid (think waffle batter in terms of consistency). It hardens to a rubbery gelatine stage after about ten to fifteen minutes.

Plaster Bandages

These are the medical bandages used to create casts (at least before modern fiberglass alternatives). They work very well for creating a quick custom mother mold. The bandages are impregnated with paster powder; dipping them in water (and squeezing off the excess) is like mixing up a thin layer of plaster. These are then applied on top of the dried alginate.

Casting Stone

This is for creating the finished positive. It can be a variety of different materials. Plaster is simple and cheap for creating a rough bust. More detail can be reproduced by using a dense and fine stone mix, such as UltraCal.

Vaseline

Vaseline (or any other petroleum jelly) is a good release agent. This is useful for keeping hairs from getting caught. (It might not be enough to cover a whole mustache, though.) It's also useful as a barrier between mold halves.