Begin by creating a two-part mold of a hand and forearm. Find a cardboard box that will allow you to stick your arm in far enough to create a fake appendage in the desired length.
Cut a U-shape down one side of the box, ending a few inches from the bottom. The U should be slightly wider than your forearm. Your forearm will rest in the U while the bottom of the mold sets.
Coat the model's hand and forearm with a generous layer of petroleum jelly.
Mix Plaster of Paris with water according to package directions.
Pour Plaster of Paris into the box up to the bottom of the U cut in the side.
Lay the model's arm palm-down in the plaster. Allow the plaster to set, which will take approximately 15 to 20 minutes. It will become very warm as it does so; this is normal.
As the plaster is setting, use a small coin to scoop out divets in the four corners of the plaster inside the box. These are registration points that will help the two halves of the mold align.
When the bottom half of the mold is set, remove the model's hand. Apply petroleum jelly to the bottom half of the mold, and reapply to the top half of the arm and hand, if necessary.
Replace the model's hand in the mold. As soon as the bottom layer is set, you can pour the top layer. Scoop another three inches of Plaster of Paris over the hand and allow to set. Plaster mixed according to package directions will be thick, so it won't run out of the U in which the model's arm is resting.
Remove the top half of the mold and allow the model to remove his hand. Leave the bottom half of the mold in the box.
Coat both halves of the mold thoroughly with petroleum jelly.
Lay the halves back together, making sure the registration points align. Wrap duct tape around the entire mold, box and all, to keep it together. Stand the mold on end so that the fingers are pointing down.
Insert a tube of latex caulk into a caulk gun and cut off the tip.
Fill the mold with caulk. Allow to set for at least 48 hours.
Remove the duct tape and carefully pull the two halves of the mold apart. This will be easy since you've coated them with petroleum jelly. Trim any excess caulk from around the edges of the arm and hand. Paint the hand with latex paint and add fake nails if desired.