I've never actually done it, but several years ago I looked into it. It's not a complicated process, but if it's a complicated shape, you have to make the mold in pieces so you can get the part out of the mold when you're done. The basic process can be as simple as coating the part you are going to copy with a mold release agent, then covering it with fiberglass . . . when it cures, pop the fiberglass off and there's your mold. Coat it with a release agent and lay fiberglass in it. When it cures, pop out the part and clean up the edges.
