Plaster body moulds are indispensable for artists and sculptors who need to work from a realistic perspective. They can be used for reference, but often times they become a piece of the actual artwork, saving the sculptor the time and effort of carving or fabricating the body part from scratch. The process in not exceptionally difficult, but it is rather time consuming, and requires an exceptionally still model, so keep this in mind as you begin.
Grease your model with a thick, heavy coating of petroleum jelly. If you do not coat the skin with petroleum jelly, you will pull out your model's body hair when you remove your mould. It will also be more difficult to remove your mould, and can cause cracking and breaking. As plaster dries, it creates a fair amount of heat, so make sure your model is aware of this before you begin. Petroleum jelly also helps protect the skin from the heat.
Fill a pan with warm water and set it close to your model. Have your model move to the desired position. Note that this process takes some time so your model will need to be in a comfortable position or able to keep very still for at least an hour, likely more.
Take your first instant mould plaster bandage (see Resources) and cut it to size. Dip it in the pan of water, so it is completely saturated. Once in the water, the gypsum in the bandage will mix with the water to create plaster. This plaster dries quickly, so you will need to be ready to place it as soon as you dip it.
Apply the bandage to your model. Smooth it into any crevices or body contours. Also smooth out any air bubbles. Repeat the wetting and application process until you have covered the entire area you wish to mould, then begin another layer. Make at least three or four layers of plaster bandages or your mould will not be strong enough to hold its shape. Allow all layers to dry completely.
Use sharp scissors or a plaster knife to cut the mould exactly in half for removal. Be extra careful not to cut your model. You can use more plaster bandages to put the mould halves back together once they're removed from the model, or use them as is.
If you are casting something that naturally moves, such as a stomach that expands and contracts during breathing, plaster it in the expanded position so it doesn't crack as it dries and your model breathes.