Here is a video that shows you how: https://youtu.be/9zlvV1tOEGo
Hope that helps!
New cushions can also be made using a higher density foam that won't break down as quickly. I normally buy foam for cushions on eBay... The foam piece can be cut with a bread knife using the old cushion as a reference for size, wrapped with upholstery batting, and then the covers are put back on. I hope this helps!
This could help you: https://thistlewoodfarms.com/how-to-fix-sagging-couch-cushions/
Hi Lydia: Get some high density foam, from Joann's if you have a coupon:)
Here are some sites that may help. Ignore the ugly fabric in the first one:)
Don't sew? Try this: https://inmyownstyle.com/how-to-make-a-no-sew-cushion-cover.html