What he does is make the mold by spreading chopped fiber glass in resin over the surface of the object using a large trowel/spatula after applying mold release. Sometimes he simply layers the object in Saran Wrap after spraying with oil to get the Saran wrap to cling to the surface. He spreads the glass fiber resin in layers until he gets a mold about 1/2-3/4" thick. He makes his own using chopped fiberglass but you can also buy something called kitty hair in gallon containers at any auto body supply shop. It doesn't matter how fast you apply it or if you let it set between layers (although he likes to let each layer set before building up the next). A thicker layer mold is better. If it is a big mold, he might lay glass mat over the whole thing and apply resin to strengthen it.
Once you have the fully set mold, release it from the object. Then set it into a wooden frame box with enclosed sides and some holes drilled on all sides. Then pump in expandable foam insulation (the stuff you get in cans at home depot) through the holes in the sides to fill in under the mold and support it. Once the foam insulation has cured, use the mold to cast the part. The low expansion foam is best so that the mold doesn't get pushed up out of the box when it cures. He will also sometimes fill the mold with water or sand before pumping in the foam to give it some weight to keep it in place.