Measure all around the outside of the cushion - the perimeter should measure about 90". You need to prepare length of binding this long, plus an extra 5" for each corner, i.e. about 110" !
Cut three 2 1/2" strips across the width of your binding fabric and sew them together on the diagonal as shown, then trim off the excess fabric and press in half lengthwise with the raw edges matching. Apply the binding as shown in step 9 of my Charm Pack Baby Quilt instructable
Once you have applied the binding you can slip your pillow form / cushion inner into the cover and lean against your creeper!