Cut a 12-inch length of board using a saw. This is the top of your stool.
Cut the remaining board in half to have two pieces about 12 inches long. These are your supports.
Mark the halfway point along the side of each support.
Cut a notch that extends halfway through the width of each support using a keyhole saw. The notches should be about 1 inch wide and 6 inches deep, and the two notches should fit into each other securely, forming an X.
Sand all three wooden pieces smooth using 100-grit sandpaper.
Fit the two supports together at the notch to form an X.
Set the flat square board on top of the X, each point of the X lining up with a corner.
Clamp the supports to the top using a C-clamp.
Hammer a line of 2-inch nails through the top and into the supports bellow, spacing each nail between 1 and 2 inches apart. This holds the top to the supports. At this point, the stool is ready to be used.