Coat the object that you would like to make fiberglass copies of in a mold release agent. If you are working from a soft clay sculpture, coat the sculpture in Krylon Crystal Clear or another clear sealer first, then let it dry before applying a mold release agent. This will protect the clay.
Mark out the dividing line of the sculpture or object, from one side of the base to the other. Along this line, build up a wall of modeling clay that is at least as tall as you would like your mold to be. This way, you can pull your plaster mold apart with minimum damage.
Mix together a small amount of gypsum and water in a bucket until you have a creamy texture. Paint this plaster mixture directly onto one side of the sculpture or object from one end of the modeling clay to the other, creating an impression coat. Let the impression coat cure for six hours.
Add another layer of plaster on top of the impression coat and, while it is still wet, add strips of burlap over the top, soaking them in the plaster. Let this coat dry, then add another layer of plaster and burlap over it. Continue on in this way, layering plaster mix and burlap strips, until your mold is as thick and strong as you would like it to be. The larger the mold, the stronger it needs to be, so you may not have to do many layers for a small object mold.
Repeat Steps 3 and 4 for the other half of the object, then let the entire mold cure for six hours.
Separate the mold halves by pulling them away from the modeling clay, carefully removing the halves from your original object. Clean any clay residue from the inside of the mold. You should now have two sides of a negative mold of your original object.
Prepare the mold for fiberglass casting. Paint the inside of each mold half, making a smooth coating that will aid in the separation of the mold from the cast. When the paint is dry, apply a mold release agent like polyvinyl alcohol. Two coats of polyvinyl alcohol is recommended. When the mold release is dry, your mold is ready for laminated resin and fiberglass casting.