Select the appropriate fabric for your patio. Water-resistant fabrics shed a great deal of water, but will soak through in heavier rains. Waterproof fabrics, meanwhile, repel all water, but are less comfortable to the touch.
Take the measurements of the existing ottoman base. Then add 2 inches to these figures. Cut two pieces of fabric, plus foam, to the new, increased measurements. This will provide accurate cushion coverage for the ottoman base.
Pin the fabric together, with the finished sides pressed together. Sew around three ends, leaving a three-quarter- to 1-inch border around the edges. When complete, turn the fabric right-side out. Fill with foam and sew the remaining edge shut.
Adorn, if desired, with piping, ribbon decoration or buttons. The completed cushion can be secured to the ottoman base with hook-and-loop strips or simply set on top.