Line the mold with a plastic mold liner or with plastic wrap. The liner makes it easier to get the plaster out of the mold.
Mix the plaster per the instructions provided with the specific type and brand of plaster you chose. In general, you will need a 5-gallon bucket, plaster mix and water. The mixing ratio of plaster to water will vary.
Fill the mold with plaster and then level the top of the plaster in the mold with a trowel. Allow 24 hours drying time.
Hold the open end of the mold in your hand and flip the mold over to release the plaster brick from the mold.