Measure the length and width of the mattress. Write the measurements down and determine how high you want the headboard to rise above the mattress.
Measure the plywood and note the measurements for the length and the height of the headboard.
Cut the piece with a table saw or circular saw.
Sand the cut edges with a hand sander.
Hold the cut piece against the wall where you want the finished headboard to be. Make a mark with a pencil on either side of the bed just under the headboard. Measure from the mark down to the floor for the leg height.
Measure on the leftover plywood two pieces that are the right height for the legs and 4 inches wide. Cut two of these pieces with the saw.
Sand the cut edges of the legs with a hand sander.
Place the front of the headboard face down and line up the legs with the bottom edges of the plywood.
Place two flat brackets over each of the seams and attach the brackets with one screw into each side of the seam.
Sand, paint or stain as desired.
Have someone hold the headboard in place at the designated site. Screw the headboard directly into the wall, or attach mounting brackets to the back.