For the legs, I wasn't extremely precise. The back end of the board should be 12" off the ground. So I cut a 2x4 11" long (taking into account the width of the OSB). One end is a right angle, and I adjusted the miter saw to cut a 12.5 degree angle for the side that touches the ground. Then I drilled a hole and fastened the legs with a bolt, one washer on each side and a nut. Clamps are very helpful in this step.