Place the 1-by-12-by-36-inch board on a work surface with one 12-by-36-inch side laying flat. This is the bench seat. Measure in from each end 2 inches and mark the measurements with a pencil. These marks indicate the location of the legs on the bench seat.
Place the 1-by-12-by-18-inch boards parallel to each other on a work surface 30 ½ inches apart. One 1-by-12-inch end of each board should be flat on the work surface. These are the legs. Position one 1-by-6-by-30 ½-inch board between the legs, one on each side. The 1-by-30 ½-inch side should be flat against the work surface. The 1-by-6-inch ends will butt up against the inside 1-by-12-inch side of the legs. These are the stretchers.
Drive two evenly spaced flush trim wood screws through the outside of the legs into the ends of the stretchers to fasten the legs to the stretchers.
Stand the legs and stretcher assembly upright so that the legs' ends opposite the stretchers are on the work surface. Align the bench seat on top of the assembly with the pencil marks facing the floor.
Align the pencil marks with the outside edges of the bench legs. Drive four evenly spaced flush trim wood screws through the bench seat into the ends of the legs. Fasten the seat to the stretchers by driving screws every 6 inches through the bench seat into the stretchers.
Fill the screw holes with wood putty. Allow the putty to dry. Sand the putty so that it is flush with the surface of the wood.
Paint the bench and allow the paint to dry.