Choose a wood headboard that is the same width as your metal bed frame. You want it to look like it could be part of the unit.
Cut a strip of wood blocking to the same width as the headboard using a saw.
Use a stud finder to find the studs in the wall behind your bed. Mark them with a pencil.
Find the spot on the wall that is a few inches below where the top of the headboard is meant to stop. Mark this location over the wall studs. Use a level to get the marks perfectly even.
Mount the wood blocking over the level area you found in the previous step, using 2 1/2-inch screws.
Position the headboard so that the wood blocking is only a few inches below the top of the headboard, or right under the cap of the headboard, if there is a cap. Attach it using 4d finish nails, with nail a few inches in on each side of the headboard.
Reinforce the bottom half of the headboard by attaching it to the lower studs in the wall, using 2 1/2-inch screws.
Slide your bed into place. The bottom of the headboard will be covered. Once you add a bed skirt and decorative quilt, the metal frame will be hidden and it will look like you have only a wood bed frame with a headboard.