Place one of the 6-by-3 feet pieces of plywood on it's side, so that the 6-foot side is on the ground.
Place one of the 2-by-3 feet pieces on it's side, with the 2-foot side on the ground. Align the 2-by-3 feet piece of plywood so that it is flush with one end of, and at a 90-degree angle to, the 6-foot piece of plywood. Attach the pieces together using three 1-inch wood screws at even intervals.
Measure the 6-foot piece of plywood along the length to find the center. Mark this spot. Place a 2-by-3 feet piece of plywood on it's side, with the 2-foot side on the ground, against the 6-foot piece of plywood at a 90-degree angle where the mark was made at the center of the 6-foot piece of plywood. Attach these two pieces using three wood screws.
Place a 2-by-3 feet piece of plywood at the other end. Align it so that it is even with the end of the 6-foot board and at a 90-degree angle. Attach with three 1-inch wood screws.
Place the second 6-foot piece of plywood so that it is parallel to the first 6-foot piece of plywood. Line it up so that it is flush with the end of the 2-by-3 feet piece of plywood that was attached first to the other 6-foot plywood board. Attach the 6-foot piece of plywood with three 1-inch wood screws.
Align the remaining 2-by-3 feet pieces of plywood, so that they are at a 90-degree angle to the second 6-foot plywood board. Attach each of them with three 1-inch wood screws.
Lay the third 6-foot piece of plywood flat on top of the frame that was just built. Align the board so that it is flush with the top and side edges of the cabinet. Attach the pieces using 1-inch wood screws at each of the corners and then at 12-inch intervals along the edges of the cabinets.
Turn the cabinets over, so that the open, front part of the cabinets is visible. Decide on the placement of the shelf for each of the cabinets, by sliding a 2-by-2 feet 11 1/4-inch board into the open cabinet area. Once the placement of each shelf is decided, secure them with two 1-inch wood screws at both sides.
Attach two hinges to each side of two 2-by-3 feet plywood boards, which will be the cabinet doors. Attach the hinges of the cabinet doors to the cabinet unit.