Set the foam on a flat surface, like the floor or a work table. Turn the bench over and place the seat area on the foam. With a utility knife, cut the foam completely around the bench, leaving a 4-inch margin.
Lift the bench off and set it to the side. Apply spray glue to the foam, then quickly set the bench on top so the foam adheres to the seat area. Leave the bench upside down on the foam until the adhesive completely dries, about 20 minutes.
Lay the fabric on the ground, right side down, then lay batting over the top. Set the padded vanity bench face down in the center. Cut the fabric and material out around the padded bench, leaving a 4-inch margin.
Pull the fabric and batting taut on opposite sides of the bench. Turn the fabric and batting underneath by one-half inch, then one-half inch again, to make a small hemmed edge. Secure the fabric with a staple gun to the underside of the bench, at least 1 inch in from the edge. Staple the opposite side in the same way. Work your way down the long sides of the bench on both sides.
Pull the fabric tight at one corner, and fold it so it makes a pleat on the top side of the bench. Trim any excess batting if the corner is too bulky. Place several staples on the corner fold to secure the fabric and preserve the pleat.
Work your way around the bench, turning the fabric underneath for the hemmed edge and pleating corners as needed. Place staples about 2 inches apart to secure the fabric all the way around on the underside of the vanity bench.
Turn the bench over and check the padded seat for any areas where the fabric is not pulled tight. Pull the fabric tight in those areas and staple.
Trim the edges of the vanity bench with upholstery nails, fringe, lace or any other accessory to match your vanity.