Plaster moulds can be made of almost any object and used to create multiple copies of it. In the beginning it is best to make moulds of simple objects. As your skill as a mould-maker increases, so too can the complexity of the objects you wish to copy. Plaster moulds can be used to cast copies out of plaster or even out of metal.
Find a cardboard box or wooden box frame with sides high enough to contain at least 2.5 cm (1 inch) of clay plus half the height of the object you are moulding plus at least 5 cm (2 inches) of Plaster of Paris. The box should be at least 3.5 cm (1 1/2 inches) wider and longer than the object you are copying. If using a cardboard box it is best to have two identical boxes. If using a wooden frame it is best not to have a bottom to the frame, and if the frame can be easily disassembled and later reassembled, so much the better.
Prepare the object that you wish to copy by covering it with a thin coat of petroleum jelly. Get the petroleum jelly into all nooks and crannies. Make sure the coating of petroleum jelly is even.
Put enough clay into the box that you can embed 1/2 the thickness of the object you are copying into the clay and still have approximately 1.25 cm (1/2 inch) of clay beneath the object. Smooth the surface of the clay but do not allow the clay to dry out. Use moist paper towels to keep the clay moist and pliable if necessary.
Insert the object you are copying half way into the clay. In other words, half of the object is embedded in the clay and half of the object is sticking up above the surface of the clay. Use your finger or the eraser end of a pencil to make four indentations near each corner of your object. These indentations will later be used as registration guides to assemble the two halves of your mould.
Make a clay rope approximately 0.6 cm (1/4 inch) thick and lay it from the edge of your box to an unobtrusive spot on your object. This will form the fill hole of your mould later on.
Mix the plaster of Paris as the manufacturer's instructions describe. The final mixture should be the consistency of thick pancake batter. Tap the buckets several times to release any air bubbles in the plaster of Paris.
Coat the edges of the box with petroleum jelly. The jelly acts as a release agent so that the plaster of Paris will not stick to the object you are moulding and the edges of your box. You do not need to worry about the clay--the plaster will not stick to the clay. Pour the plaster of Paris into the box so that it covers the object being moulded and is a minimum of 2.5 cm (1 inch) thicker than the object. 5 cm (2 inches) thicker than the highest point of the object is even better. Allow the plaster to cure as described in the manufacturer's instructions.
Remove the cured plaster from your box. If using a cardboard box this may involve tearing the box away from the mould as well as the clay. Use a thin-bladed knife to gently pry the plaster mould off of the clay. Turn the mould over and gently work out the object you are moulding. Take your time and rock the object back and forth until it finally releases.
Turn the mould over and place it back into the wooden box or into a cardboard box of the same size as the original. Place the object to be moulded back into the half mould you made and cover the mould, the object you are copying and the sides of the box with a coating of petroleum jelly. Fill the box with plaster of Paris and allow to fully cure. Using a thin-bladed knife, pry the two halves of the mould apart and gently work the object that you are copying out of the mould. After coating both halves of the mould with petroleum jelly you can now set the two halves of the mould together (using the registration "pins" you made with your finger or the eraser end of a pencil) with rubber bands or bungee cords, and pour plaster of Paris or even molten metal into the fill hole you created. Once the copy has set you can pry apart the two halves of the mould and you should have an exact duplicate of the original item you wanted copied. All you will need to do is to sand off the protrusion caused by the fill hole and touch up any small imperfections.
Allow the plaster of Paris to fully dry during each stage in order to avoid breaking it when pulling it apart.
Always cover all release points with an even and complete coating of petroleum jelly.