Stand up two 3/4-by-18-by-36-inch plywood boards on their long, thin sides to be the side boards. Set a 1-by-8-by-24-inch plywood board against either end of the side boards. Position 1-inch metal L-brackets around the inside of the frame, spacing the brackets every 5 inches between each joint.
Drill holes into each bracket hole with the drill. Attach the brackets to the frame with 3/4-inch wood screws.
Set a 3/4-by-24-by-38-inch plywood board on the ground. Position the frame on top of this board, which is the rear board of the cabinet. Set metal brackets around the frame against the frame and the rear board. Drill out each bracket hole, and attach the brackets to the frame and the rear board with 3/4-inch wood screws.
Set a 3/4-by-24-by-37-inch plywood board on top of the frame. Set two side-mount cabinet hinges underneath the door and above the side board. Mark the locations of the hinge screw attachments, lifting the door if necessary to access the holes. Remove the hinges and drill out all marked spots. Attach the hinges to the door and the side board using 3/4-inch wood screws.
Position a doorknob on the edge of the door board opposite the hinges, and mark its location. Drill through the door and slide the doorknob bolt through the hole. Fasten the doorknob nut on the other side.
Place a magnetic 2-part latch under the door and against the side board. Mark and drill out each screw hole. Attach the latch to the door and the side board with 3/4-inch wood screws.
Position a 3/4-by-18-by-22 1/2-inch board within the cabinet to be used as a shelf. Set 1-inch metal L-brackets every 12 inches underneath the shelf and against the rear and side boards. Attach the brackets with 3/4-inch wood screws. Repeat as needed for additional shelving.
Sand all cabinet surfaces with fine-grit sandpaper. Wipe the cabinet clean with a rag and apply a coat of paint or stain with a brush, making the brush strokes in the same direction as the wood grain. Let the first coat dry for two hours then apply additional coats until the right color tone is reached. Apply at least two coats of sealant with the same method. Let the cabinet dry overnight before installing.