Prepare plaster using two parts water and one part plaster in a bucket. If using concrete, mix one part Portland cement with three parts gravel and two parts sand. Add water to the mixture. The proportions of components in concrete may vary, but mix until obtaining the consistency of yoghurt. Make clay slip by adding water to clay and mixing until you obtain a liquid solution.
Place your mold on an even surface. Wet the inside of the mold with a sponge, to yield fewer bubbles. Place the mold with the hole facing up.
Pour the liquid plaster, cement or clay slip to fill the mold. Use small buckets of material and pause between each bucket to allow the material to settle. Shake the mold to eliminate bubbles.
Allow the material to harden. Plaster hardens within two hours, while concrete may need between four and 12 hours, depending on the size of the sculpture and the room temperature. Clay slip may take up to 48 hours to dry. Check if the material is dry by inserting a finger or a paintbrush in the hole of the mold. If the material sticks to your finger, the sculpture is not hardened.
Remove the sculpture from the mold.
Sand the sculpture using fine-grit sand paper for clay and coarse-grit sand paper for plaster and concrete sculptures.