Remove the mirror from the wooden frame. It will often be held in place by clips that need to be loosened, or it might be held in place by staples.
Cut two strips of leather, one for the top and bottom of the frame. Measure around the top strut of the frame with a tape measure to determine the width needed. Make each strip wide enough to stretch around the frame and meet at the back.
Apply glue to the top strut of the frame, gluing the front, top and back. Don’t apply any glue to the inside of the frame yet. Lay the leather over the frame again, adjusting where necessary. Press the leather down over the front of the frame, working outward from the center so it lies flat without any air bubbles, then along the top and the back. Leave to dry.
Cut the bottom section of the leather, which will be hanging into the frame, to the width of the interior of the frame, then glue in place, wrapping the leather around to the back of the frame. Staple the leather in place on the back of the frame for extra hold. Leave to dry.
Lay a straight edge so it makes a diagonal line from the inner top corner of the frame to the outer corner. Cut along this line with the knife top give a mitered (angled) corner, then repeat for the other corner.
Repeat the gluing and cutting process for the bottom of the frame.
Cut two leather strips to the outer length of the side of the frame, making them wide enough to wrap around the frame. Apply glue to the back, outside and front of one side of the frame, ensuring that you only glue between the mitered edges of leather already in place at the top and bottom of the frame.
Put the leather strip in place and press down. Leave to dry.
Cut a mitered corner very carefully where the leather at the sides meets the mitered corners at the top and bottom, ensuring that the pieces of leather meet without a gap. Work from the outside of the frame to the inside.
Change to a straight, not an angled, cut where the leather meets the inside of the frame, then pull the leather round the frame tightly. Staple in place on the back of the frame.
Replace the mirror in the frame.