To frame a bathroom mirror, cut molding or casing to match the mirror, cut 45-degree miters on each end using a miter saw, join the pieces with construction adhesive, and attach the frame to the outer edges of the mirror with the same adhesive. If desired, fill the gap around the edge of the frame with caulking.
Measure the length and width of the mirror, and purchase four lengths of white casing or molding with 2 or more inches of excess to allow for angled edges. Miter each end at a 45-degree angle so the pieces create a tight frame when the edges meet. Check for gaps before gluing the mitered edges together using a strong construction adhesive, and hold them together until dry using painter's tape.
Use putty or caulking to fill any leftover gaps between the mitered edges, and run a finger over the seam to smooth the putty. Apply two coats of paint in your desired color. Once the paint has dried, run two parallel beads of construction adhesive along the back of the frame at least 1 inch from the outer edge. Gently press the frame onto the mirror until the adhesive sets, and check to ensure it's level before caulking the gaps between the frame and the bathroom wall.