Cast paper is made by pressing paper pulp into a mould of some kind while wet and letting it dry in the mould. I have experimented with different kinds of moulds - plaster of paris, drywall compound, polymer clay, and regular clay. My favorite is the polymer clay - I use Sculpty. It dries fairly flexible which helps in removing the dry paper. Once removed, I play with the casts using paints, pastels, and shoe polish (a recently discovered favorite).