Cut a 20 inch length piece of the 1-by-12-foot board using a handsaw. This is the top of the stool. Label the piece and set it aside.
Cut a 13 inch length piece from the 1-by-12-foot lumber. This is a leg for the stool. Use this leg as a guide to cut a second leg. Label both legs.
Cut two sides for the stool from 1-by-8-foot lumber, each measuring 17 inches long. Label these as the sides.
Set both legs upright on a flat surface so they are sitting on one of their legs. Arrange both legs so they are parallel with each other and 17 inches apart.
Spread an even layer of wood glue over both ends of one of the side pieces. Slide one side board you cut in between the two legs. Adjust the board so it is flush with the sides and the tops of the legs. Repeat on the other side with the second side board. Allow the glue to sit for a few minutes, but do not allow the boards to slide out of place.
Attach the sides securely to the legs using wood screws. Screw through the sides of the legs and into the ends of the sides. Use three wood screws at each joint.
Place the top piece on top of the stool. Adjust the piece until it is perfectly center, you can use the measuring tape if necessary. Screw the top to the side and leg pieces. Use two wood screws on each side of the top.
Rub medium-grit sandpaper all over the stool. This removes all splinters and rough edges.
Paint the stool with an even coat of wood stain. Allow the stain to dry before moving on to the next step.
Fill in cracks and screw holes with wood filler. This gives the stool a finished look.