Lay two 1/2-by-15-by-37-inch boards on the working surface to be the sides of the cabinet. Arrange the boards be 13 inches apart, resting on their long, thin sides and parallel to each other. Push a 1/2-by-13-by-15-inch board, the top board, against one end of the side boards. Line up the boards so their outside edges are even with each other, then drill three pilot holes through the top board and into each side board. Put 2-inch wood screws into each pilot hole, and fasten the boards together by tightening the screws.
Repeat with another 1/2-by-13-by-15-inch board on the other end of the side boards. Attach it with 2-inch wood screws to make it the bottom board of the cabinet.
Rest a 1/2-by-13-by-38-inch board on top of the cabinet frame to be the rear board. Drill pilot holes through the rear board and into the cabinet frame, spacing them every 6 inches. Set 2-inch wood screws into each hole in the rear board and tighten them down with a drill.
Rotate the music cabinet and set another 1/2-by-13-by-38-inch board on top of it to function as the door. Set two side-mount hinges between the board and the cabinet. Mark the position of the hinge holes and drill them out without breaking through the surface of the door. Screw 1/2-inch wood screws into the hinges to attach them to the door and cabinet.
Set the cabinet upright on its bottom board and position a 1/2-by-12-by-15-inch board within the cabinet in a desired shelving position. Mark the sides of the cabinet at that level, then drill two holes into the insides of both shelf boards along that marking. Set pegs in the holes and position the shelf board on the pegs. Repeat as needed for additional shelving.