Create a template for your headboard. You can use a large sheet of cardboard. Trace out the shape you want the board to be in and then cut away any surplus cardboard.
Trace out the shape onto the plywood. Lay your cardboard template over the plywood. Make sure you are tracing on the "A" side of the board — the back will be much harder to work with. Use a light pencil to make your marks.
Cut the plywood to shape. Use a circular saw and slowly follow your pencil marks. Do not rush the wood through the saw; doing so will create jagged marks.
Sand down the plywood. The edges will need to be sanded smooth and the "A" surface will need to be prepped for painting. Use a fine grade of sandpaper and go over the surface once. The edges may need a medium grade, depending on how jagged they are.
Paint the plywood. Paint the wood the desired color. Go with the grain and apply one thin coat of paint. Don't forget to paint the edges of the board too. Let this first coat dry for at least four hours. If you need a second coat, apply it and then wait another four hours before installing the headboard.
Attach the headboard. Use a drill to make holes in the bottom of the board to attach it to the frame of your bed. You can use screws for a wooden bed, but a metal bedframe will need nuts and bolts. Make sure the headboard is sturdily attached.