Lightly sand the wood surface of your headboard. Your goal is to ready the wood to accept new paint, not to remove the finish that's on the headboard. If the furniture has a wax finish, sand the wax finish completely away. If you're not sure, lightly sand and prime a small test section at the foot of the headboard. If the primer adheres well after a few days, you can lightly sand the headboard.
Clean the sanding debris with a damp cloth. Make sure to remove all of the debris; whatever you leave will show up when you repaint.
Inspect the headboard for nicks or gouges. Repair any you find with wood putty. You can apply wood putty with a putty knife, similar to icing a cake. The putty knife helps you level the putty with the surface of the headboard. Allow the putty to dry according to the manufacturer's instructions.
Apply a coat of primer to the headboard. It's a good idea to prime before you paint, especially with old wood as the primer helps the paint go on smoother. If you are repainting your headboard in a dark color, have your primer tinted to match. This is help to eliminate one topcoat. Allow the primer to dry according to the manufacturer's directions.
Apply a coat of paint to the surface of the headboard. Smooth, even strokes in the direction of the wood grain will create a better looking finish. Follow the directions for dry times on the container so you know when you can add another coat or add the topcoat.
Inspect the headboard after the first coat of paint dries. If you notice splotchy areas or if you can see the primer through the paint, add another coat of paint. Let the paint dry.
Inspect the headboard again. Two coats of paint are generally enough for most projects, but if your headboard needs another coat, apply one. Let it dry. When you are satisfied with the appearance of the headboard, add a final topcoat of polyurethane to protect the finish.
Apply the polyurethane with a paint sponge. A paint brush will often show brush strokes in a glossy or semi-glossy finish.