Open the flip-open top panel of the cabinet. It is typically hinged on one side of the cabinet to open sideways, like a book. Use a screwdriver to remove the hinges, and save the hinges and screws. Set aside the cabinet top panel.
Remove the sewing machine and the hardware that attaches it to the cabinet. The sewing machine may have wing nuts anchoring it, or you may need to use a screwdriver to unfasten it from the cabinet.
Clean out the enclosure under the sewing machine space. Newer cabinets have metal, plastic or wood protective enclosures that surround the machine when it is in its closed position in the cabinet. Antique cabinets may not have an enclosure.
Measure across the rear edge of the top of the cabinet and pencil-mark positions to replace the hinges so the top panel opens with a front-lift motion. Use the original hinges as guides. Measure and mark corresponding hinge positions on the top panel. Drill holes for the hinges, and attach the top panel to the cabinet.
Measure and mark the position for the toy chest prop-lock. Drill and install the lock along one side edge of the cabinet and top panel.
Remove the top panel by unscrewing the hinges and prop lock, and place it with the inside surface facing up. Measure the center, and mark it to position the mirror and mirror flat-mount hardware. Attach the mirror to the top panel with the hardware. Replace the top panel on the hinges and prop-lock.
Make an enclosure if your cabinet does not have one. Cut plywood or laminate board and assemble it into a topless box that fits under the machine space. Attach it under the cabinet to the base using L-brackets and screws.
Sand any unfinished wood. If the original hinges left flush-mount marks, fill them with wood putty and sand it smooth after the putty is dry. Paint or finish the raw areas to match the cabinet. Repaint or refinish the entire cabinet if the original surface is marred.