Locate a flat, level area to construct your foam pit, preferably an open field with ample room to add a ramp and trail. These will allow you to build up enough speed for your approach.
Deploy your chalk line to mark off a rectangular area for construction of the pit. The foam pit should measure 50 feet by 100 feet at minimum. Use the wooden stakes to mark the four corners of the chalked rectangle.
Build your foundation for the foam pit with cinder blocks. This will ensure stability and will raise the pit floor at least 6 inches off the ground to allow water drainage.
Build a solid floor. Use the 4-by-4s for the frame, then apply the 1-inch plywood sheets for the floor surface.
Build the walls of the foam pit. Use 4-by-4s for the skeleton, 2-by-8s for the mid-supports, and plywood sheets for the wall surfacing. The walls should be at least 12 feet high to allow placement of sufficient foam padding to absorb impact.
Drill drainage holes in the floor. Place them every 3 feet or so and make them about 5 inches in diameter. Drill between the support beams, not over them.
Cut your mesh screen into circles a bit larger than the holes you drilled. Place them over the drainage holes to keep out critters seeking shelter. Consider double-screening the holes to ensure durability.
Fill the pit with foam. You can purchase customized foam cubes that will total anywhere from $25,000 to $50,000, depending on the size of your pit. Or you can go to junkyards and get foam from junked mattresses. Be careful not to shred the foam; cut it into cubes after cutting away the mattress casing from the foam. Do not attempt to pull the foam out of the casing; this will cause shredding.
Build the trail and ramp. The trail requires nothing fancy, just a simple dirt path free of obstacles and long enough for the bike to accelerate adequately. You can purchase a motocross ramp rather than build your own, which will require some sophistication. The ramp involves the use of wood and piping to build and demands expert welding abilities. Use the metal piping for the frame and incorporate the wood as the ramp surface, providing traction. Plan the ramp to measure 8 feet high, providing enough lift for aerial maneuvers.
Build a crane to pull your bike out after each jump by using the winch, the steel I-beam and steel pole. This will require expert welding, so you may need help from a professional welder. Another option is to rent a crane each time you practice. This is more expensive over the long term.