Use an assistant to help hold two 1/2-by-18-by-60-inch plywood boards on their long, thin sides. Keep the boards, which are the side boards of the pantry, parallel to one another with about six inches between them. Hold the boards in place and set a 1/2-by-6-by-18-inch plywood board against one end of the boards. Adjust the new board, which is now the top of the pantry, so its outer edges are even all around with the side boards.
Drill four evenly spaced holes down through the top surface of the top board and into the ends of the side boards, for a total of eight holes. Set 1 1/2-inch wood screws in each hole and tighten them securely to firmly attach the boards together. Repeat on the other end of the side boards with another 1/2-by-6-by-18-inch plywood board to serve as the bottom board.
Place a 1/2-by-6-by-61-inch plywood board on top of the rectangular frame of the pantry so its outer edges are even with those of the pantry. Drill holes down through this board, which is the rear board of the pantry, and into the frame every 12 inches. Set 1 1/2-inch wood screws in each hole and tighten them completely to install the rear board.
Give the pantry frame a 180 rotation so the rear board is now flush with the working surface. Settle another 1/2-by-6-by-61-inch plywood board on the open frame of the pantry. Slide four side-mount cabinet hinges between this board, which is the door of the pantry, and one of the side boards of the pantry. Space the hinges evenly and mark their screw hole locations on the side board and door, lifting the door slightly if necessary.
Drill out each hole marked for the hinge screws, and reposition the hinges. Attach the hinges to the door and side board using 1/2-inch wood screws placed in the holes and tightened securely.
Stand the pantry cabinet upright and position a 1/2-by-5-by-18-inch board within the pantry at a desired shelving height. Drill two holes through each side board and into this small board, which is a shelf board. Set 1 1/2-inch wood screws in each hole and tighten them securely. Repeat as needed for additional shelving.