Measure your mattress. Standard U.S. sizes are: California King - 72-by-84 inches, King - 76-by-80 inches, Queen - 60-by-80 inches, Full - 54-by-75 inches and Twin - 39-by-75 inches. For the purposes of this article, a queen-size mattress is used for the platform bed.
Cut two 2-by-6-boards to 58 inches long. Cut two more boards to 81 inches long. Lightly sand all edges to smooth any rough spots.
Measure 1 inch down on the inside of the boards and mark with a pencil.
Cut two 2-by-2 boards to 58 inches long and two to 75 inches long. Align the top of the 58-inch board to the top of pencil line on the 58-inch, 2-by-6 board. Pre-drill and screw together, using the 2 1/2-inch screws. Center the 75-inch board on the 81-inch, 2-by-6 board, aligning to the top of the pencil line. Again, pre-drill and screw together.
Lay the boards in a rectangle, butting the pieces together in the corners and carefully aligning the tops. The 58-inch boards fit inside the 81-inch boards. Pre-drill two holes in the sides of the 81-inch boards and into the ends of the 58-inch boards, 1 1/2 inches from the top and bottom. Use the lag screws to screw the frame together.
Cut the plywood panels to 29-by-78 inches. Lay side by side inside the top of the frame. Pre-drill and screw down to the 2-by-2 boards, using the 1 1/2-inch screws.
Cut the remaining 2-by-6-inch boards, two at 58 inches and two at 78 inches. Lay in a rectangle with the 58-inch boards inside the 78-inch sides, tops aligned and ends butted together. Pre-drill and screw together, using the lag screws. Screw an "L" bracket into each corner, using the 1 1/2-inch deck screws. Smooth any rough edges with sandpaper.
Place the smaller rectangle in position in the bedroom, 2 inches from the wall. With a helper, lift the top of the platform bed onto the base, with the inner 2-by-2 supports resting on top of the 2-by-6 base.
Place the mattress on the platform and make the bed.