If you would like to purchase one instead of make one then just check out our store, http://www.sewstylishboutique.etsy.com/ .
First you need to get the right cloth. You should always use 100% cotton fabric, quilters weight or home decor weight.
You need a yard and a 1/4 of fabric. If you want a wider cover to decrease the risk of exposing get yourself a yard and 1/2.
D-rings, you can find them at craft stores
16" of boning, you can buy at Joann's
First pre-wash you fabric and iron. Next measure a 3 1/2 inch strip along the bottom width of your fabric and cut. Measure 14 inches from that strip and cut. You now have 2 strips one 14" the other about 29". Fold the strips right sides together, pin and sew a 1/4" seam allowance leaving the ends open.
Turn the strip right side out using a safety pin turning it in towards and pushing it threw the strip of fabric.
Press the seam open. Fold the raw end in on one side of the long strip and top stitch closed.Do not finish the other ends on the strips. You will see why.
Cut 16" of boning and push a half inch of the plastic top and snip it then tuck it back in the cloth casing.Center the boning on the mark on your fabric edge lay the boning flat and pin in place on .
Now fold the straps up and pin. Sew along the seam over the straps, not catching the boning in the stitch.