Cut off a 12-inch segment from a 1x12-inch board using a saw. This is the seat.
Cut off two 4-inch segments from the same 1x12-inch board. These are your two legs. The stool you are making will have a profile like a wide, flat staple.
Place one leg standing up on its long side.
Place one edge of the seat on top of it, raising the opposite edge with scrap lumber.
Hammer a line of nails 1 to 2 inches apart along the edge of the seat, attaching the top of the seat to the leg.
Place the second leg underneath the side opposite the first leg. This time, the stool is supported on the first leg.
Hammer a line of nails 1 to 2 inches apart into the seat again, attaching the second leg.
Sand the stool thoroughly using 100-grit sandpaper. At this point, the stool is ready to use.