Sturdy tension rod
Polyester quilt batting
Install a sturdy tension rod between the headboard's two posts (left). Ensure the fit is tight.
Measure the entire width of the headboard, including the outer edges of each post (the batting will go around the posts, too). Measure the height of the headboard; double it. Cut a swath of polyester quilt batting (buy it at fabrics stores) to these dimensions.
Drape batting over tension rod and posts, allowing it to fall evenly over the back side of the headboard. Whipstitch the front and back sides together along the left and right edges to hold in place.
To create the slipcover, measure the width of the headboard, again to the outside edges of each post. Add 2 inches to this measurement to allow for 1/2-inch-wide side seams and room for the batting. For length, measure from the top of the tension rod to the bottom of the headboard. Add 2 inches to this measurement for the top seam and hems. Cut two pieces of your fabric to these dimensions.
Place right sides of the fabric pieces together; pin along top edge and the two side edges using 1/2-inch seam allowances. Fit the pinned cover over the headboard and tension rod to check the fit. Adjust if necessary. Stitch together along these three edges.
Sew a double 1/2-inch hem along the bottoms of the front and back pieces. Turn right sides out. Fit the cover over the headboard and batting.