Use a tape measure to measure the length and width of the futon you'll be using on your Japanese bed.
Use the futon length measurement to cut two frame pieces that will create the sides of the platform. Use boards that are 4 by 6 inches to create a platform 6-inches tall. Use the width measurement minus 8 inches to cut three boards the width of the futon for the platform. These three you will place at the head and foot of the platform as well as in the middle to add some strength to the frame. Cut the plywood with a circular saw to match the length and width of the platform.
Use a carpenter's square to square up the frame on the floor, making sure all angles are 90-degrees. Clamp the boards in place with bar clamps to hold them securely during the frame assembly.
Drill two holes through every joint where the frame pieces meet. Use a 1/2-inch bit and make the holes 6-inches deep. Secure the parts together with lag screws through the joints, tightening the screws flush with the wood using a wrench.
Remove all of the wood clamps, and then place a bead of wood glue over the top of the frame. Place the plywood over the frame, covering it completely so that the edges are flush with the frame edge. Weigh down the board with bags of sand, heavy books, or other weighted objects and allow the board to dry for 24 hours.
Remove the weights, and then lay out the futon centered onto the platform for your low Japanese style bed.