Sand the surface of the wood, starting with rougher 800-grit sandpaper. Sand until the surface of the headboard has a consistent smoothness.
Graduate to 1000-grit sandpaper and sand the headboard until the board is smooth all over. Given the finer grit, the headboard will feel smoother than when you sanded with the 800-grit sandpaper.
Sand the surface of the headboard with 1200-grit sandpaper until the headboard has a consistent smooth feeling. Again, the headboard will feel smoother than when you sanded with the 1000-grit sandpaper.
Wipe the surface of the headboard down with a tack cloth to remove any fine dust particles left from the sanding process.
Apply a small amount of semi-gloss latex paint to a lamb's wool paint roller and then roll the paint onto the headboard. Use a wide "W" pattern with the paint roller and add additional paint to the roller as needed. Wait 24 hours and then add a second coat of paint.
Place a paper stencil onto the front of the headboard if you plan to add a stenciled design. Tape the paper onto the board if the stencil does not have a sticky back.
Paint over the stencil with a semi-gloss latex paint of a different shade to add contrast. Wait 24 hours for drying and then remove the stencil from the headboard.