Rip the seams with a seam ripper. Open the material where the cushioned back of the couch's seating area meets the fabric that covers the flat back. This seam us usually directly across the highest point of the couch and it opens the fabric panel. Do this for each cushioned backing panel if you have more than one.
Cut pieces of cotton batting with scissors and stuff this into the cushioned area until it is as full as you like. Do this for each area you opened with the seam ripper.
Thread an upholstery needle with heavy-duty thread that matches your couch fabric.
Sew the panels back into place. Use the old needle holes as your guide. Knot the end of the thread when you finish the seams.
Fluff the backing of the couch to move the cotton batting around. If any areas seem lumpy, push on the fabric like you are fluffing a pillow.
Unzip all cushions and remove the foam inserts.
Pull off the worn layer of foam batting with your fingers. The layer is likely adhered, so it may come off in pieces. This should leave you with plain foam cushion inserts.
Spray one side of each foam cushion with spray adhesive and lay a sheet of cotton batting over each.
Cut the excess batting from around each cushion and flip the cushions over.
Repeat the process of applying spray adhesive, laying cotton batting and cutting the excess away. Allow the adhesive to set for up to about 30 minutes.
Replace the cushions into the covers and zip.