Put on a dust mask to avoid breathing in any plaster particles. Wear disposable gloves, as plaster can irritate skin.
Turn on the hot water tap to fill a container with warm water. Test the water temperature to make sure that it is between 70 and 100 degrees Fahrenheit. Weigh out your plaster and water. The proper proportions are between 74 and 82 parts water to 100 parts plaster.
Pour the proper amount of water into the bucket. Use your hands to sift the plaster evenly into the bucket of water. Let the plaster soak into the water for one to two minutes before mixing.
Submerge your hand in the bucket to combine the plaster and water. Break up any lumps between your fingers. Continue to mix until the mixture has a smooth, even consistency.
Tap the bucket to release any air bubbles. Carefully pour the plaster into your mold or casting container.
Press an object into your plaster-filled casting container to leave an impression. Do not submerge the item more than halfway or you will not be able to remove it without breaking the plaster apart.
Allow the plaster to harden before disturbing your mold or cast. If desired, you can carve plaster when it is semi-hard or hard with clay tools.