Place the two 1-by-12-by-18-inch boards parallel to each other on a work surface. Space the boards 22 1/2 inches apart from the inside edges. Stand the boards upright so that they are resting on the 1-inch ends. These are the stool legs.
Lay the 1-by-12-by-24-inch board flat across the top ends of the stool legs. This is the stool seat. The board should rest against the top of the stool legs with the outside edges of all three boards flush.
Drive four evenly spaced pan-head wood screws through each end of the seat into the top ends of the legs with a screw gun to fasten the seat to the legs.
Center the 1-by-3-by-22-1/2-inch board between the inside edges of the legs. This is the cross support board. The top edge should be flush against the bottom of the seat board. Drive two evenly spaced pan-head wood screws through the outside of both legs into the ends of the cross support board to fasten it to the legs.
Apply two even coats of wood finish to the stool assembly with a paintbrush. Allow the first coat to dry four hours and apply the second coat. Wait 24 hours after the second coat has dried to use the stool.