Remove the existing light fixture, including mounting brackets. IMPORTANT: turn off the power to the light fixture (breaker box and switch) before you begin! Working with electricity can be dangerous! If in doubt whether you possess the necessary skill and experience, always contact a professional!
Apply adhesive to the back of your medallion. Although some adhesives reduce the need for screws, drill some pilot holes in your medallion before applying adhesive. Prepare screw anchors as necessary so you can securely attach the medallion to your ceiling. Make sure your adhesive is suitable for the material of both the medallion and the ceiling.
Attach the medallion. Press it firmly in place and secure using screws. Make sure all the wires and connecting pieces for your light fixture are easily accessible (IMPORTANT: Double check power to the light fixture is shut off!).
Apply caulk around the edges of the ceiling medallion to seal it.
Cover your screws. Fill in any screw holes with spackle and paint over them, if necessary.