Coat your finished clay sculpture with Krylon Crystal Clear or another clear sealing agent, so that you get the smoothest possible surface and will have an easier time removing the soft clay from the mold. If you are working from a hard sculpture or wish to save your original sculpture, coat the sculpture in Vaseline or a release agent instead, so that the mold does not stick. Let the coat dry.
Place your sculpture in a protected workspace, with all surfaces covered. Mark out a center line running from the base of the sculpture, to the top and back down to the other side of the base, and build up a wall of modeling clay there. This clay will separate the mold into two halves, so that you can remove it more easily from your sculpture. If you do not care about the original soft clay sculpture getting ruined, make your mold in one part.
Mix up a small amount of gypsum and water in a bucket until you have a soft, creamy texture. Paint the gypsum mixture onto one side of your sculpture, creating a smooth base coat with as few bubbles as possible. Let the base coat dry, then mix up a larger batch of gypsum in the bucket and apply a second coat. Atop the wet second coat, add a layer of burlap strips and paint a third coat of gypsum over them. This will make the mold sturdier. Continue adding layers of gypsum mixture and burlap until your mold is hard and strong. Let the mold cure for six to eight hours before continuing.
Pull each half of the mold off your sculpture slowly, removing the modeling clay as you go. If necessary, you can sand down the inside of the mold so that it is smoother. Strap the two halves of the mold together, using a piece of rope or a series of rubber bands, and flip the mold over so that you can pour your casting material directly into the mold. You may need to build a frame to hold the mold upright, or lean it strategically against another object.