Ah, the dreaded couch-cushion sag. One day you're sitting proud, the next you're sinking into a fabric abyss, lost to the world. Fear not, for you can fight back against this furniture tragedy! While the sources you provided were about as helpful as a screen door on a submarine, I can certainly guide you on this noble quest for plumpness.
First, you must perform a cushion autopsy. Bravely unzip that cover and peer inside to see what you're up against. Is it a single, sad block of foam? A chaotic jumble of polyester fluff (poly-fil)? A depressing collection of what used to be feathers? Knowing your cushion's innards is the first step to victory, as it determines what you'll need for the revival.
Next comes the shopping trip. For cushions with a foam core, your best bet is to buy a new piece of high-density upholstery foam and some quilt batting. The foam provides the structure, and wrapping it in a layer or two of batting adds that plush, welcoming softness. If your cushion was just loose fluff, you can buy giant bags of fresh poly-fil. Just grab more than you think you'll need; you want to stuff that cover until it's firm.
Now for the main event: the great unstuffing and restuffing. If you're replacing a foam block, use the old, flattened piece as a template. A pro tip is to cut the new foam about an half-inch to an inch larger on all sides. This "overstuffing" is the secret to a full, professional look. Wrap your newly cut foam in the batting, using a bit of spray adhesive to hold it in place. Then comes the final boss battle: wrestling the new, plump insert back into the cover. It helps to fold the foam like a taco to get it started. Shove it in, let it expand, and then patiently adjust until it sits perfectly. If you're using loose fill, just start cramming it in there, making sure to push it firmly into every last corner. Don't be shy; those corners are where future sag begins.
Zip it up, give it a triumphant karate chop in the middle to settle everything, and place it back on the couch. You have successfully vanquished the sag. Now go enjoy your resurrected throne; you've earned it.
