Prepare the panel door for use as a headboard. Take off hinges and any other hardware. Sand any rough spots and paint or stain to coordinate with the decor of your bedroom.
Place the panel door horizontally on the floor or other flat work surface. Determine what side of the door you want facing outward as the headboard.
Install one D ring into the door 2 inches down from the top and 2 inches in from the side on the right. Position the D ring so the flat part runs parallel to the top of the door. Use a drill to form starter holes for the screws if necessary. Screw the D ring in place.
Repeat the D ring installation process in the parallel location on the left.
Measure the width between the two D rings. Mark this measurement on the wall above your king bed to determine the location for hanging the hardware. Place the marks at the height you want the headboard.
Run a stud finder over the area of the marks. If studs are present, install two picture hooks, each with a 50-pound capacity. If no studs are present, drill pilot holes for 1/4-inch toggle bolts in the wall, using a 5/8-inch drill bit. Squeeze the wings of the toggle bolts together and insert into the holes until they open. Tighten the bolt, leaving it extending from the wall several centimeters.
Mount the panel door headboard to the wall by lining the D rings up with the bolts or picture hooks and slipping them over.