Lay the plywood--cut to your specifications--on a flat surface. Measure 6 inches in from each corner and mark it.
Center the mounting brackets on the marked spots. Attach all four brackets to the plywood using the enclosed screws.
Screw in all four legs. Be sure they are secure.
Stand the ottoman upright. Glue the foam to the top of the ottoman. Allow to it dry.
On a flat surface, layer the fabric (wrong side up) and the batting. Be sure the materials are centered. Turn the ottoman upside down. Center the foam in the middle of the batting. Fold the batting over the edges of the foam and plywood. Staple the batting to the plywood.
Fold the fabric over the batting and staple it to the plywood. Be sure the fabric is taut but not pulled. Keep the material smooth and even as you staple. The corners can be overlapped or folded if necessary.
Cut off any excess material beyond the staples.
Stand the ottoman upright and enjoy.