Arrange four 1/2-by-3-by-24-inch tongue and grooved boards next to each other so they are locked in place. Cut off the visible tongue of the end board using a saw. Apply wood glue to the remaining tongues and stick the boards together firmly. Suspend them in a position to dry without sticking to anything for at least four hours.
Surround the glued boards with two 1/2-by-2-by-16-inch boards and two 1/2-by-2-by-24-inch boards so that the four boards form a rectangular frame around the glued boards. Drill two holes at each frame board joint, through the side of one frame board and into the ends of the adjoining frame board. Attach the boards together by setting a 3-inch wood screw in each hole and tightening them fully with a drill.
Drill one hole on each side of the door through each frame board and into the central tongue and groove boards for just four holes total. Attach the frame to the central boards with four 3-inch wood screws tightened completely.
Set two 1/2-by-18-by-27-inch boards on their long, thin sides on the working surface. Arrange these boards, which will be the side boards of the cabinet, parallel to each other about 16 inches apart. Position a 1/2-by-16-by-18-inch board against one end of the side boards. Arrange this board, which will be the top board of the cabinet, so that its ends are even with the outside surfaces of the side boards.
Drill four holes through the top board and into each side board. Attach the boards together with 2-inch wood screws placed in each hole and tightened completely. Repeat to add another 1/2-by-16-by-18-inch board on the other side to be the bottom board.
Set a 1/2-by-16-by-28-inch board on top of the rectangular frame of the cabinet. Drill holes through this board, which will be the rear board, every eight inches through it and into the rectangular frame. Secure the rear board to the frame by placing 2-inch screws in the holes and tightening them fully.
Flip the frame over. Set the completed farmhouse door on top of the cabinet frame. Align two evenly spaced side-mount cabinet hinges along the side of the door and the cabinet. Mark their hole locations and drill the holes out. Attach the hinges to the door and the cabinet using 1/2-inch wood screws.