Measure the perimeter of the mirror. Purchase extra frame molding to allow for errors.
Cut the molding for the mirror frame. Use a miter saw and miter box to cut the correct angles, which should all be about 45 degrees. Another option is to use four corner blocks on the mirror. Cut the molding that connects the four medallions straight across, using a hand saw. Butt the edges together.
Test-fit the molding around the mirror. Make any adjustments in angles, as necessary.
Finish the molding with paint or stain, following manufacturer’s directions. Also finish a scrap of molding. Allow the paint or stain to cure for at least two weeks. By waiting, there is less chance of a chemical reaction between the paint or stain and the mirror’s surface.
Mix a small amount of clear epoxy according to package directions. Apply the epoxy to the rabbet edge of the piece of scrap molding. Press the glued edge to the edge of the small piece of cut glass. Hold the molding in place with a piece of painter’s tape. This is your test run to see how well the epoxy is going to hold the molding to the mirror. Allow the epoxy time to cure before removing the tape. Check the bond between the molding and the glass to see if you need to use more or less epoxy as you attach the mirror frame.
Mix enough epoxy to do one side of the mirror frame at a time. Apply the epoxy to the rabbet edge and place it against the mirror. Hold the molding in place with painter’s tape. Continue adding the pieces of the molding until you have the entire frame in place. Allow the epoxy to dry completely, following the recommended time on the package.