Purchase one or two plaster ceiling medallion molds. These can be purchased at a hardware store or online and are reusable. Often these molds are made of plastic and come with instructions that tell you how much plaster is needed.
Cover your work area with newspaper. Spray the inside of the plaster mold with Airid mold spray or brush it with mineral oil.
Mix some plaster of Paris in a mixing container following the directions on the plaster bag. Mix until smooth, but be careful not to overmix it, as the plaster will set too quickly.
Starting at one edge, slowly pour the plaster into the mold. Tap the mold lightly to remove any air bubbles. The mold will feel warm as the plaster sets, which usually takes 15 minutes.
Right before the plaster has set, scratch grooves on the exposed surface. This will allow for better adhesion between the ornament and ceiling.
After the plaster sets, place your hand over the opening of the mold and gently turn it over. The plaster piece should easily slide out. Allow the plaster pieces to cure for a couple hours to a few days, depending on the size of the piece.