A queen headboard is not as wide as a king-size bed, so if you want to attach it to a king-size frame, the mounting brackets will not line up with the corners of the bed frame. Headboard adaptor plates enable you to attach a smaller or larger headboard to a bed frame.
Remove all bedding, the mattress and the box spring from the king-size bed and set aside.
Hold up the queen headboard next to the head of the king bed frame and mark the sides of the queen headboard on the bed frame with a chalk marker. The chalk marks the left and right side edges of the headboard where it will attach to the frame. Remove the headboard and set aside.
Place a headboard adaptor plate on the head of the frame and mark two of the slots with a chalk marker on the frame. The adaptor plate has several holes in it to accept screws and may turn horizontally or vertically to attach the headboard to the frame. Turn the adaptor plate so it is horizontal with the right edge of the chalk mark on the right side of the frame.
Drill a hole through the frame on each of the two marks. Place the adaptor plate on the head of the frame over the two holes. Screw two screws into the frame to hold the adaptor plate. Repeat this procedure for the second adaptor plate on the other side of the bed.
Hold the queen headboard up to the adaptor plate and attach two screws to the headboard with a screwdriver on each side of the headboard.
It is helpful to have a second person to hold the headboard up to the frame for marking and screwing the headboard onto the bed frame.