Coat your model deer with a mold release agent, unless you are working with a clay model. A clay model can be coated with a clear sealer like Krylon Crystal Clear.
Seal the inside of your wooden mold box with modeling clay, so that the plaster does not stick to the wood.
Mix together 2 cups of plaster of Paris with 1 1/2 cups of water, or as much as needed for your project size.
Pour the plaster of Paris into the mold box until it has filled enough to hold your model. Let the plaster sit for 2 full minutes.
Press your deer model into the plaster of Paris, but do not fully submerge it. Make sure that the flat back side sits level with the plaster.
Let the plaster dry for 12 hours. Pull the deer model out of the mold and take the mold box apart.
Follow Steps 1 through 3 in the previous section. Fill half the mold box with plaster of Paris and let it sit for 2 minutes.
Press the deer model into the plaster of Paris, letting the plaster cover only half of the model. Let the plaster dry for 12 hours, then spray the dried plaster with mold release.
Make another mixture of plaster of Paris and pour it into the mold box, covering the deer model completely. Let the plaster dry for 12 hours.
Take the mold box apart and separate the plaster deer mold halves. Remove the deer model.
Carve a pouring hole into the plaster using a craft knife. Make sure that the hole reaches the cavity inside the mold. Tie the mold halves together with rubber bands.