Measure the width of the bed frame and cut the plywood (or have it cut to size at the hardware store) to the measured width by 40 inches--a full size mattress is typically 54 inches wide, but your frame may be slightly larger. Cut the top edge into a curve with a jigsaw, if desired.
Unroll the batting and spread it evenly across the plywood. Wrap the batting around the edges of the plywood and staple the ends to the back of the wood.
Iron the fabric and place it over the batting. Center the pattern, if applicable. Staple the ends of the fabric to the back of the headboard.
Wrap a piece of scrap fabric or a rag around the head of a hammer and pound nail head tacks or decorative buttons into the fabric. Space the trim or buttons evenly across the headboard or around the edges.
Find and mark the framing studs behind the bed. Hold the headboard up to the wall in the exact location where you want it mounted. Make sure it is level and mark the wall around both top corners with a pencil.
Screw mounting brackets with the slides facing up into two separate studs behind the bed, close to the top of the frame. Measure the wall to determine the distance between the corners of the headboard and the mounting brackets. Transfer the measurements to the back of the headboard and screw the other half of the brackets into place, slides facing down.
Hold the headboard up to the wall and slide the mounting brackets together to secure the headboard. Erase any visible marks on the wall.