Align two of the square boards to form a corner and nail into place. Add the next board to begin to create a box. Add the fourth board and finish the box shape.
Set the base board on top of the box and nail to the box frame along the sides, not the corners. The corners will be where the casters attach. Any misplaced nails may prevent the casters from attaching properly.
Mount the casters by pre-drilling holes in the corners of the base square. Screw in the casters and secure tightly so they will not pop off under pressure and movement.
On the final square, lay out the four 1-inch boards in a square shape, 2 inches away from the edges of the large square. Nail into place, creating a lip to keep the top from sliding off the ottoman.
Lay out your fabric square, right side down on a flat surface. Center your foam over the fabric followed by the lid over the foam with the 1-inch boards facing up. Be sure everything is centered with equal amounts of fabric all around.
Pull the fabric tightly over and around the foam and lid to the 1-inch board side and staple in place. Keep the tightness consistent to maintain a smooth flat look on the fabric. Flip the top back over and place onto the ottoman to finish.