Measure each edge of the mirror molding using a measuring tape.
Cut four pieces of pre-finished wood trim corresponding to the edges of the mirror using a saw. Choose a wood trim that is at least as wide as the molding.
Bevel both ends of each of the four strips at a 45-degree angle. This allows the four pieces to fit together like a picture frame.
Apply a thin layer of wood glue to the bottom of one of the strips and press it to a corresponding molding. The wood strips should hide the molding entirely.
Secure the trim to the molding with several strips of masking tape.
Repeat steps 4 and 5 for each wood strip.
Allow the wood glue to cure completely. The amount of time this takes is referenced on the back of the glue’s packaging.
Remove the masking tape.