Find a model object, or create a model statue of your own out of Sculpey clay. If you choose to make your own, sculpt the clay and then bake it in the oven, following the packaging directions. Sand the hardened Sculpey smooth.
Make a molding box for your object. Build a square of Legos that you can fit your object into, giving it at least 1/4 inch of extra space on all sides, top and bottom. Flatten a piece of modeling clay and press the Lego square onto it, so that it functions as a bottom. Leave the top side open. If your object is very large, you may need to build a molding box out of wood.
Mix up a batch of silicone RTV rubber, adding the catalyst agent to the rubber as directed on the packaging. Pour the rubber into the molding box, filling the box halfway.
Press your object into the rubber until it is covered up to its halfway point, then press wooden crafting balls around the object in the same way. Cure the rubber overnight, or for at least 12 hours.
Take the wooden crafting balls out of the rubber, revealing the keying depressions. Mix up another batch of silicone RTV rubber and pour it into the box, over the object and into these keying depressions, all the way up to the top of the molding box. Let the rubber cure overnight again.
Take the molding box apart and pull the mold halves away, removing the original object from the center. Fit the mold halves back together, matching up the keying depressions, and strap them together with rubber bands.
Cut a hole in the bottom side of the mold with the razor blade, so that the inner cavity is accessible.
Mix together the polyester casting resin with its catalyst, following the directions for your specific brand of resin. Make sure that the catalyst is evenly mixed into the resin and that it is free of air bubbles.
Pour the resin mixture into the hole in the bottom of the mold, filling the mold slowly all the way to the top. Let the resin cure for 5 hours, or as directed on your product's packaging.
Pull the rubber bands off the mold and separate the mold halves, removing the cast statue from the middle of the mold.
Sand the resin statue smooth by holding it underwater while you work with the sandpaper, so that you are not exposed to the resin dust. If you would like to sand the resin out of the water, you will need to wear a particle ventilator. Paint and finish your resin statue to your liking.