Four ¾×1½×23¾-inch trim boards
Two ¾×1½×12-inch trim boards
Two ¾×1½×13½-inch trim boards
White satin paint
Dowels & trim pieces in various styles and sizes from ½ inch to 2 inches wide
Miter box with saw
Center mirror on particleboard using level and measuring tape. Mark corners with pencil. Apply joint compound to the back of the mirror. Set mirror in marked position and apply slight pressure. Let dry.
Paint precut trim boards with white paint. Let dry. Arrange the 23¾-inch pieces around the perimeter of the particleboard with the ¾-inch sides facing outward, keeping the edges of the material flush with each other. Use a nail gun to a secure the tops of the trim boards to the outside edges of the particleboard. Place the 12-inch trim boards on the side edges of the mirror, making sure mirror and trim are flush at the corners. Secure with nail gun. Add 13½-inch pieces to remaining edges of the mirror, aligning corners. Secure with nail gun.
Cut a mix of dowels and trim pieces to 23 inches and 9½ inches long using a miter box and saw (number needed will be determined by the widths of your pieces). Paint pieces white; let dry. Arrange pieces as desired inside the frame. Use nail gun to secure the pieces to the particleboard. Board should be completely covered.
For a more finished look, fill nail holes with joint compound and touch up with white paint. Let dry.