Decide on the layout for the heavy plaster medallion. Recruit a friend to help you lift the medallion to the ceiling. Trace around the medallion using a pencil.
Locate the ceiling joists with an electric stud finder. Mark the joists with a pencil. Transfer the placement of the ceiling joists to the plaster medallion, as well.
Hold the medallion in place---with the help of one or two friends---and drill pilot holes through the medallion and into the ceiling joists. Create marks on the plaster medallion, so you can easily align it after you have applied to glue to the back of the medallion. Remove the medallion from the ceiling.
Apply construction adhesive to the back of the ceiling medallion.
Place the ceiling medallion on the ceiling, and secure it in place through the pilot holes, using coated screws.
Allow the construction adhesive to dry for 24 hours. Caulk the edges of the ceiling medallion. Fill the screw holes in the plaster with no-shrink joint compound.