Set two 1/2- by 12- by 36-inch boards parallel to each other with the help of an assistant, setting their long, thin sides on the working surface. These will be the side boards. Position a 1/2- by 12- by 24-inch board -- which will be the cabinet's top -- against the side boards perpendicularly so that it touches one end of both boards. Adjust the top board so that its ends are even with the outside surfaces of the side boards.
Drill three evenly spaced holes through the top board and into each of the side boards. Drive a 2-inch wood screw into each of those holes and tighten thoroughly so the boards fit snugly together. Repeat at the other end with another 1/2- by 12- by 24-inch board, which will be the cabinet's bottom.
Set a 1/2- by 24- by 37-inch board on top of these boards, and make sure the edges are even around the frame. Drill holes every nine inches through the new board, which is the rear board, and into the frame of the cabinet. Drive a 2-inch wood screw into each hole and tighten.
Flip the frame over and set another 1/2- by 24- by 37-inch board against the frame. This board will be the cabinet's door. Slide two side-mount cabinet hinges into even positions against the door and the frame. Raise the door if necessary to mark the positions of the hinge screws and drill them out. Set 1/2-inch wood screws in the holes and fasten them down well to connect the hinge to the door and the frame of the cabinet.
Repeat the previous steps with the exception of the door attachment. Position a 1/2- by 12- by 23-inch board within the cabinet opening and mark its location on each side board.
Drill two holes into each side board at the level of the marking. Push wood pegs into each hole.
Set the shelf board on top of the pegs and repeat as desired to add more shelving.