That depends on the material you'll be casting. If it's fiberglass, the glass cloth will be applied to the mold surface making a thin hallow casting. If you're using resin try slush casting. Mix up your resin and pour it into the mold. Then roll the mold around by hand, allowing the resin to coat the mold surface. Keep it moving to keep the resin thickness as even as possible. It may take several applications to get the thickness you want.
If you have a two piece closed mold, roto casting would be the way to go. It's like slush casting, but done using a roto machine. Lots of threads here on that subject, including ideas on how to build one.