By Adalheid Voigt. Sofa. At Wednesday, March 20th 2019, 22:42:33 PM.
If you're unable to reach up into the corners, a tool or something like a ruler wrapped with a soft towel will help to send the fill up to round out any sunken edges. I used a cordless telephone. True story. Add filling until you can't add anymore, and then add more. Continue to add fill as you back your hand out of the cushion, making sure there is a decent amount down by the zipper -you don´t want what you stuffed up in the top of your cushion to all settle to the bottom.
Wrap the foam with multiple layers of batting. A small amount of spray adhesive applied to the foam can help the batting stick to it so it stays in place once you put the covers back on. If possible, have someone help you put the covers back on the cushions. All those layers of batting make it a tight fit- but that is what you're looking for, so ask someone to lend a hand to make your job less painful.
My mom was pretty positive there were no zippers to access the seat backs on her sofa, but after a little spelunking she was very surprised to find that, indeed, there were hidden zippers along the bottom back sides. If you can only get to the seat cushion, skip the Poly-fil and just pick up batting. For seat back access only: just Poly-fil.