Measure the width of the mattress, and add 8 inches for the size of a headboard. Most home improvement stores will cut the plywood for you at no cost; otherwise, cut the plywood to size using a jigsaw.
Spray the plywood with spray adhesive. Position the quilt batting flat on the floor, and center the plywood over the batting. Pull the batting to the back, and staple 2 inches from the edge of the plywood. Start at the top-center, followed by the bottom-center, left-center and right-center. Move from the center toward the corners, stopping when you are 6 inches from each corner.
Trim the batting at the corner to remove the bulk. Overlap the batting, and cut away from the corner at a 45-degree angle. Remove the excess batting. When you staple to the corner, you should not overlap any batting fabric at the corner. Finish stapling. Trim any excess batting from the back side along the staples.
Place your decorator fabric face-side down on the floor. Position your headboard over the fabric, with the batting-side down. Center your headboard over the fabric. If your fabric has a pattern, align the pattern before attaching the fabric. Use the same stapling pattern used for the batting. Stretch the fabric slightly so the front is smooth and taut.
Fold the top fabric, and tuck the ends into the side fabric. With thin fabrics, you may be able to fold tight enough to create a straight vertical fold with the side fabric over the tucked top fabric. Usually, you will need to make a second smaller tuck to create the straight fold that lines up with the top horizontal line of the headboard. Trim away excess fabric to make the fold as crisp as possible. Staple the corner when the fold is horizontal. Repeat for your remaining three corners. Finish stapling, and trim off your excess fabric.
Disguise the lumpy or uneven corners on a headboard by using trim applied over the existing fabric cover. Measure around the circumference of the headboard, and add 6 inches. Hammer tacks into the corners to pin down and flatten out the fabric bulk as much as possible. Use as many tacks as you need.
Cut grosgrain ribbon that is the thickness of the edge of the headboard to your length measurement. Hot-glue the ribbon to the edge of the headboard, starting in the bottom center. Keep the edges even and crisp. Overlap your start point, and trim off the excess ribbon.
Cut trim the same length as the ribbon. Hot-glue the trim over the center of the ribbon to add dimension and to disguise any discoloration of the ribbon.