Use a tape measure and the 1-by-2 wood piece to measure two 16-inch pieces and two 20-inch pieces. Mark your measurements on the wood with a pencil.
Cut the pieces from the 1-by-2 wood piece with a miter box and hand saw. Cut the ends at 45 degree angles.
Assemble the pieces together to form a rectangular frame. Check that the 45 degree angles are joined tightly.
Use a staple gun to attach the pieces together at the back where the angles meet. Three staples are sufficient to hold the pieces together.
Measure two 16-inch and two 20-inch pieces on the quarter-round wood trim. Mark your measurements with a pencil. Cut them with the miter box and hand saw with 45 degree angles at the ends.
Place them on the front side of the wooden frame on the outer edges. Check that the rounded lips of the wood pieces face inward. Attach them to the wooden frame with headless nails to form the trim of the frame. Four nails on each side are enough to securely hold the trim in place.