If you just want padding, cut shape of your headboard out of any kind of sturdy material, cover it with a thin piece of fabric (like an old sheet) then lay a piece of thin foam over that, another piece of sheeting, then a layer of batting, sheet fabric, then a layer of fabric you want to show. Staple around back. Make a French cleat apply one half to back of headboard, other half to wall. Foam can be any thickness you want.
Hello. This Headboard upholstery Link might be helpful.