Cut a 3/16th-inch wide groove that is 1/2 inch deep down the center of one of the long, skinny sides of both of the footboard top/bottom boards using a table saw. Slide the beadboard into the groove between both boards.
Position the side boards next to the assembled footboard so the top of the side board is flush with the top of the top board and so are the bottoms. Make any adjustments necessary so the side boards are the same size as the assembled footboard. Make two evenly spaced marks on the front surface of each joint between the top/bottom boards and the side boards.
Predrill a centered hole into the end of the top and bottom boards at the location of each marking. Predrill a centered hole into the side of the side boards at each marking. Insert pegs into the holes in the side boards and slide them into the holes in the top and bottom boards. Repeat for the other side of the footboard.
Glue the pegs once the entire footboard has been assembled properly by removing the side boards one at a time and putting wood glue on each end of the pegs. Allow six hours for the wood glue to harden. Repeat the entire process for the headboard.
Position the support boards on the front surface of the headboard, 1 foot off the ground. Predrill four evenly spaced holes through the support board and into each of the side boards in the headboard. Attach the support board to the headboard with 2-inch wood screws. Attach the support board to the footboard with the same method.
Stand up the head and footboards with the help of an assistant and position one of the side boards between the two, with the end of the board flush against the head and footboards and the side of the board flush against the support boards. Predrill three screws through the side of the side board and into the end of the support board. Attach with the 2-inch wood screws. Repeat for the other side board.
Arrange the mattress support boards evenly on top of the two side boards. Predrill two holes down through the top of the mattress support boards and into the side boards. Attach the mattress support boards with 2-inch wood screws.