Repeat at all four corners. Remove fabric from carton, straighten any pins if necessary or remove pins and take a row of basting instead. Sew down each of these lines on the sewing machine. Everything should fit smoothly and snugly. If you feel there is too much excess fabric at the corners remove the cover and cut off extending fabric.
Return cover to position.
Turn footstool upside down, bring fabric around to the back and tape with duct tape to hold. To tidy things up cut a piece of fabric slightly smaller than the bottom of the footstool, hem it around all four sides and hand stitch it to the bottom of the footstool covering all the raw edges. Cover the carton as described but without adding the pillow effect padding on top.