Cut six 16-inch lengths of 2-by-4-inch board with a circular saw. Cut three 4-foot pieces of 2-by-4-inch board.
Place the three boards onto the floor, broad side down. Place one 16-inch board at each end of the boards, facing upwards, so the boards describe a very shallow and wide "U" shape. Screw the boards together where they intersect, with two wood screws at each spot.
Lift the first U-shaped frame up to the rafters of the basement ceiling, with the aid of an assistant or two. Slide the uprights of the frame 2 inches past the edges of the rafters on both sides. Screw the frame to the rafters with two screws at each location.
Place the second and third frames in place in the rafters, each spaced 4 feet from the other. The first and third frames should then be 8 feet apart. Attach each to the rafters.
Slide the sheet of plywood onto the bottom portion of the frames. Nail the plywood to the boards it rests upon.