Set up a work station in a well-ventilated room. Cover your flat work surface with plastic trash bags, taped down to the surface to prevent them from moving.
Put on your safety equipment. Wear a vapor respirator, safety goggles and latex gloves to prevent contact with the toxic resin and its fumes. Make sure that your sleeves cover the bare skin of your arms.
Coat all of the inside pieces of the 3-part silicone mold with mold release.
Place the core piece of the doll mold between the outside halves of the mold. Secure the entire mold together with a few rubber bands.
Pour your polyurethane casting resin into a plastic cup and add the catalyst, following the packaging directions for your brand. Mix the resin and catalyst well.
Set the doll mold onto your work surface, with the pouring hole facing upward. Pour the casting resin directly into the hole, filling the mold completely.
Let the resin cure completely, for at least one day. Repeat steps 3 through 7 on any other doll molds you may be using for your doll project.
Open the mold and take the completed doll cast out of it. Swap your vapor respirator for a particle respirator and sand the surface of the cast until it is smooth.