"Many fossils form by a natural process that involves a mold being made and then filled with a substance which hardens into rock." In this demonstration you "use clay as the substance from which the mold is made and plaster as the rock."
Coat a shell with Vaseline. Roll a lump of clay larger than the shell flat. Press the shell into the clay hard enough to leave an imprint. Pull the shell out of the clay. Mix a small amount of plaster according to the directions on the package. Pour the plaster into the clay well. Allow the plaster to harden. Once hardened, peel the clay off the plaster. This is called a cast of the shell.
source: Exploring Creation with General Science by Dr. Jay Wile