Hey there! It's a real bummer when your once-plump sofa cushions start to look a little sad and saggy. Fortunately, this is usually a pretty easy fix you can do yourself.
While the provided YouTube context wasn't relevant, the Reddit source points to the most common and effective solution, which involves the cushion's inner filling.
The first thing you'll want to do is get inside the cushion cover. Most back cushions have a zipper that makes this simple. If yours doesn't, you'll have to carefully use a seam ripper to open up a small section of a seam, preferably on the bottom or back where it will be less visible later.
Once you have access to the filling, the simplest fix is to add more stuffing. You can buy a bag of polyester fiberfill (often sold under the brand name Poly-Fil) at any craft or fabric store. Before adding the new material, use your hands to break up and fluff the old, compressed filling. Then, begin adding the new fiberfill, making sure to mix it in with the old stuffing to avoid lumps. Really work it into the corners to get a full, even shape. Keep adding filling until the cushion is as firm and plump as you'd like.
If the old filling is in really bad shape, you might opt to replace it entirely. Just pull out all the old material and refill the cover completely with new fiberfill. Once you're done, zip the cushion closed or carefully hand-sew the seam you opened. This simple refresh can make your sofa look and feel so much more comfortable
