Excavate the sod from around area of the planned shed. Fill the area with gravel or crushed rock to a depth of about 4 inches. Level the gravel with a shovel and rake.
Place two 6-by-6-inch treated lumber beams on the gravel oriented along the 20-foot length. Use two 10-foot beams on each side. Place the beams 8 feet apart. Use a 4-foot carpenter's level to level the beams. Add or remove gravel under the beams until they provide a surface that is level from end to end of each beam and level between the beams.
Cut seven 2-by-4-inch boards to 141 inches. These are the floor joists. Cut two 2-by-4-inch boards to 12 feet for the end joists. Cut two 2-by-4-inch boards to 8 feet and two to 12 feet. These boards are used for the outside frame along the long side of the shed floor.
Nail one of the 8-foot boards to the end of one of the end floor joists. Nail the two 12-foot frame pieces to the other end floor joist. Use two 16d nails for each end.
Place one of the 141 inches between the two 8 foot pieces so the center of the board is 16 inches from the center of the end stud. Use two 16d nails at each end. Continue to add the 141 inch pieces to the frame until you reach the end of the 8-foot frame.
Place the 12-foot frame pieces abutting the 8-foot frame. Nail a 141-inch piece between them so that piece abuts with half of each frame segment. Drive two 16d nails into the joist from each of the frame ends.
Place the rest of the 141-inch joists between the framework every 16 inches.
Place the floor joist assembly on top of the beams so that the frame extends 2 feet on each side of the beams. Toe-nail nails through the joist at an angle into the beam to hold the floor joists in place using 16d nails.
Place 1/2-inch plywood over the floor joists. Place the plywood so the long edges of each sheet rest on a floor joist. Nail in place with 8d nails every 8 inches along the seams and every 12 inches in the center portions of the plywood.