Measure all four sides of the ottoman. Cut a piece of material of this width, plus 48 inches to allow for 12-inch box pleats at every corner. Cut the material to the length that you desire for the skirt.
Pin the fabric to one side of the ottoman, attaching it to the slipcover. Leave 12 inches of free fabric at the corner and insert three pins: one to mark where the fabric meets the corner and another two spaced at 6 and 12 inches beyond, respectively.
Remove the fabric from the ottoman and form the box pleat by tucking the material at the first and last pins underneath, to meet at the middle pin. Smooth out any creases in the fabric and pin the pleat in place. Pin the material back to the slipcover and repeat the pleats with the next two corners.
Form the final box pleat by pinning the skirt to the slipcover and pin either side of the remaining corner. Flatten the ruffle loop that this creates to resemble the other three pleats and pin it into place.
Remove the slipcover from the ottoman. Sew the skirt to the slipcover and sew the pleats as you have pinned them.