Lay a piece of upholstery foam over the area you want to cover. Trace out the shape you need to cover the space and cut along your traced template with a utility knife. For added cushion, consider layering your sheets of upholstery foam.
Place your foam on your sofa. Apply a layer of spray adhesive to the top of your foam and cover with an even layer of cotton batting. Pull the batting over to the underside of your couch and staple in place directly to the frame. Add as much cotton batting as desired, but keep the sections of your couch even so the end result does not look lumpy and misshapen.
Lay a piece of muslin cloth over the surface area you want to upholster. Use a pencil to trace the shape you need. You will use this muslin cloth as your pattern for your upholstery fabric. Fold your muslin in half vertically and adjust to make your pattern shape symmetrical.
Lay your muslin flat and add 2 inches around each side and cut out your pattern. Pin your pattern to your upholstery fabric, print side up. Trace along your muslin template and cut out your pattern.
Place your upholstery fabric over the part of your couch you want to cover. Pull over the underside of the sofa and staple in place directly to the frame. Leave ¼ inch excess fabric around your staples and cut away any additional material.
Walk your fabric inch by inch around curved areas and the corners of your couch. As you fold your fabric, pull taut and staple in place. If your fabric is thick, you may need to cut 1-inch-long slits around the edge of your fabric to help it lay flat in place.
Add any decorative piping or welting cord around your seams. You can attach it with a hot glue gun, but for extra stability you may want to stitch your piping in place with a needle and thread.