Lay your two 1-inch-by-3-inch-by-20-inch boards on edge, parallel, and 18 inches apart. Lay your 18-inch boards between them so they are perpendicular to the 20-inch boards, and 18 inches apart. Drill two 3/32-inch holes through each end of the 20-inch boards and into the 18-inch boards. These pilot holes will prevent the screws from breaking when you screw these boards together. Drill a 3/8-inch hole that is a 1/4-inch deep into the 3/32-inch hole on the 20-inch boards. These are counter sunk holes that will hide your screw heads. Screw the 20-inch boards to the 18-inch boards so you have a 20-inch-by-20-inch frame. Drill pilot holes, and counter sunk holes for the screw heads for every screwing application in this project.
Screw your 15-inch boards to the inside corners of your frame so the edges are all flush with the same side of the frame. You will need four screws for each corner. These are the legs for your table, and they will need to be even so your table can sit flat.
Position your table upright, and screw the two 1-inch-by-10-inch-by-20-inch boards to the top of your table frame. These edges of these boards should be flush with the sides of the frame. Apply your wood putty to the 3/8-inch holes surrounding the screw heads, and let it dry. Sand down the table with your medium and then fine grade sand paper, wipe it down with your tack cloth, and apply your wood stain.