Purchase a square or rectangular unframed mirror.
Measure each of the sides of the mirror with a measuring tape.
Cut 4- to 8-inch wide window framing with a miter saw. Cut the edges of the framing for the sides of the mirror with a 45-degree outward angle. Cut the edges of the framing for the top and bottom of the mirror with a 45-degree inward angle.
Apply window adhesive along the back of the pieces of framing for the sides of the mirror. Set the pieces of framing in place, pressing on them to secure them to the mirror. Wipe away any excess adhesive that spreads onto the surface of the mirror with a damp cloth.
Apply window adhesive along the back of the pieces of framing for the top and bottom of the mirror. Set the pieces of framing in place so that the edges butt up against the edges of the side pieces of framing. Press on the pieces of framing to secure them to the mirror. Wipe any excess adhesive that spreads onto the mirror with a damp cloth.