Place the wood slat on a saw horse and cut it using a circular saw. Cut eight 5-foot-long pieces, four 4-foot-long pieces and 12 3-foot-long pieces.
Make two rectangles from the wood slats by screwing two 3-foot-long and two 5-foot-long slats together using the screwdriver.
Make a box by screwing the two rectangles together at the corners using four 3-foot-long slats. Screw one 3-foot-long slat to one of the corners of the rectangle, then screw the second rectangle to the other end of the 3-foot-long slat. Repeat this until you have screwed all 3-foot-long slats to the corners of the rectangle.
Screw the 4-foot-long slats to the four bottom corners of the rectangular box. Connect the end of the 4-foot-long slats to the four top corners of the rectangular box by screwing the remaining 5-foot-long slats on the frame.
Screw one 3-foot-long slat in the center of the two 5-foot-long sides of the rectangular box to give the frame additional strength. You now have the BMX jump box frame ready.
Screw the three plywood boards onto the frame, one on top of the rectangular box and the remaining two on top of each ramp.