Obtain a large noninflatable kiddie pool. The bigger the better, but make sure it is at least 4 feet deep with a 15-foot diameter. Don't ever go less than this depth. You can find a cheap pool without a water pump at major department stores, yard sales and Internet auction sites.
Obtain enough foam to fill your pit. Put up signs, posters, fliers and Internet ads to request donations. Ask for used foam, mats and old furniture made with foam. Talk to neighbors, schools and check out the local Goodwill store. A cheaper, but not free way, is to go to your local gym and ask if you can buy their old foam.
Lay the mats below the pool to add a protective layer between the pool and the hard ground. Cut a few holes in the pool to drain water if your pool will be outside. Fill up the pool with the foam. Categorize your foam into categories of firmness. Layer the pool with the most firm foam at the bottom of the pool and the softest on top.
Line up your ramp next to the pit. Think about the trajectory of the ramp and where you will land after the jump. Place the ramp carefully to ensure that you will land in the foam pit. The pit will not help you if you don't land in it.