Measure the distance between the current floor and the ceiling, using a measuring tape. Leave about 7 feet, 6 inches of space to comply with most building codes.
Clean the garage floor with a broom. Any dirt or debris will become trapped in the polyethylene sheeting.
Lay out the 6mm polyethylene sheeting on top of the garage floor to protect the wood from moisture. Overlap each sheet by about 6 inches and tape the seams so that nothing can get through. Connect the edges of the sheeting to the garage floor with a caulk gun and caulk to complete the seal.
Lay out the sleepers around the sides of the garage. Fasten them to the garage floor with 2¼-inch masonry nails and a hammer.
Lay out more sleepers in one direction across the room. The sleepers should be 16 inches apart, when measured from the middle of the board. Drive a masonry nail at the end of every board and every 4 feet after that, using the hammer. As you lay out the boards, make sure everything is level before driving the nails through the board. Use shims to prop up the boards if you need to make them level.
Place the plywood sheets above the sleepers and running across the room in the opposite direction as the sleepers did. Drive 6D nails into the plywood and sleepers every 6 inches around the edge of the floor and every 12 inches along the sleepers in the middle of the room.
Place whatever finished flooring you want on top of the new raised floor.