Mark off an approximately 8-inch rectangle at the end of each beam with a pencil, and then draw a diagonal line going from left to right through the rectangle. Cut through the diagonal line with the electric saw. This will leave you with a trianglular-shaped bottom and make it easier for the sled to slide along the grass without catching.
Lay the 8-foot 4 by 4s parallel to each other. Place the two 4-foot 4 by 4s between the 8-foot beams, one at the top and the other halfway down.
Connect the beams by drilling two wooden screws at each juncture.
Stand one of the 4-foot 2 by 2s up and screw it in to the top right corner of the 6-foot 4 by 4. Measure halfway across the 6-foot beam and screw another 4-foot beam into it. Do the same at the top left corner with the remaining 4-foot beam.
Wrap five layers of foam around each of the 4-foot tall beams so they're covered completely. Wrap tape twice around between each layer. As an extra precaution, tape a square of foam to the top of the beams so they're not exposed.