Set two 1/2-by-6-by-24-inch side boards with their longer, skinnier sides on the working surface, parallel to each other. Lower an 1/2-by-18-by-24 inch rear board on top of them and adjust the boards until the edges on the outside of the side boards line up with the outside edges of the rear board, and the ends of all three boards are even. Use a drill to make evenly spaced screw holes through the rear board and directly into the side boards. Space the holes every 8 inches and fill the holes with 1-inch wood screws tightly fastened.
Set a 1/2-by-18-by-6 1/2 inch top board against the open top surface of the three boards so that the edges of the top board match the edges of the other three adjoining boards. Create holes along the edge of the top board and into the other boards, spacing them every 8 inches. Set 1-inch screws in the holes and tightly fasten them. Repeat this for the bottom board.
Rotate the cabinet and set it on the working surface with its rear board facing down. Position a 1/2-by-17-by-6 inch fixed shelf board within the cabinet, four inches up from the bottom board. Hold it in place and drill two holes through the side boards and into the fixed shelf board. Fix it in place with 1-inch wood screws through the holes.
Set a 1/2-by-18-by-20 inch door board against the front top portion of the cabinet so that the door board is even with the outside edges of the top and side boards and even with the bottom edge of the fixed shelf board. Open the door as if it were already attached and position two hinges evenly in place. Mark the location of the hinge attachment screw holes. Drill 1/2-inch holes in each marking, being careful not to break through the front of the door. Attach the hinges to the door and cabinet using 1/2-inch wood screws.
Position a knob at the most convenient location on the front of the door and mark the location. Drill a hole through the door at this marking and attach the knob to the door using the supplied screw or bolt.
Position a 1/2-by-17-by-6 inch shelf board inside the cabinet at the desired height and mark its location on both side boards. Drill two holes in each side board at this marked level and insert the pegs in the new holes. Set the shelf on top of the pegs and repeat for any desired number of shelves.