Cover the plywood board with the upholstery batting. Wrap the edges of the batting around the sides of the board and pull tightly.
Staple the batting to the back of the board where they are not visible, starting in the center of the top of the board. Next, place a staple in the center of the bottom of the board, then a staple in the center of the left side of the board, and a staple in the center of the right side. Continue to staple, working away from the centers and pulling the batting as tight as you can. Space the staples 3 or 4 inches apart. When you get to each corner, fold the edges as if you were wrapping a gift. Trim any excess batting once it's all stapled.
Cover the plywood with the fabric and repeat the stapling process. Trim any extra fabric when finished stapling.
Screw one half of the flush mounts into each corner of the back of the headboard.
Position the other half of the mounts on the wall and screw in place. Use a level to make sure the wall mounts are evenly positioned. You want the top mounts to be between 24 and 30 inches above the mattress and the bottom mounts to be 6 to 12 inches below the top of the mattress. If the top edge of the headboard is 24 inches above the mattress, put the bottom mounts 12 inches below the top of the mattress. If the top edge of the headboard is 30 inches above the mattress, put the bottom mounts 6 inches below.
Hang the headboard by sliding the mounts together so that they interlock. Push the bed against the headboard.