Measure the width of your bed. Use the circular saw to cut the door so that, when lying on its side, it will fit along the width of the bed. Cut off a thin strip along the hinge edge of the door if there are screw holes. The doorknob edge will be hidden by your mattress.
Sand the entire surface of the door, front and back, until smooth.
Prime and paint the door. If your door has raised panels or panes, try using two complementary or contrasting paint colors. You can stain the door for a country look, or apply a faux finish by applying paint with a bunched cloth or sponge.
Determine the hanging height for the headboard by having a helper hold the door in place. Use the level to ensure the headboard is hanging evenly. The bottom 6 inches of the door should hang beneath the mattress. Attach the door to wall studs with wood screws and the drill.