A headboard mounting bracket is a metal or plastic piece you attach to your bed. Depending on the model of bracket, you can then slip the headboard into the bracket or bolt it to the bracket itself. While some bed frames come with pre-drilled holes for the manufacturer's brand of headboard brackets, it is possible to install a universal bracket directly to your box spring.
Remove the mattress from the box spring and set it to the side. Remove any bed skirt you have between the mattress and box spring. Turn the box spring over so it is upside down.
Place the first headboard bracket on one of the top corners of the box spring, which is either of the corners that are closest to the wall when the box spring is in its proper position.
Drive screws through the holes in the bottom of the bracket and into the box spring frame using an electric screwdriver. Repeat with the bracket on the other side of the box spring and flip the box spring back over onto the bed frame.
Slide the headboard between the box spring and the wall so the legs touch the bracket. Use the electric drill to drill a hole through the headboard legs at the level you want to attach them to the brackets. Insert the included bolt through the headboard legs and into the bracket slots, and secure them with the included nut.