Hold up and center the headboard and the head of the bed frame.
Mark where the screw holes on the bed frame's brackets line up on the wood headboard, using the pencil.
Drill 1/4-inch-diameter pilot holes through the headboard at the marked points.
Drop washers onto the shafts of button-head, hex-socket cap screws that are 2 3/4 inches long by 1/4 inch in diameter.
Thread the screws into the pilot holes, from the back of the headboard, and into the holes in the bed frame brackets.
Thread nuts onto the shafts of the screws to secure the bed frame and headboard together. Hand-tighten the nuts.
Insert the tip of an Allen key into the socket on the screw head to hold it in place. Finish tightening the nut with the adjustable wrench.