I got the seat back insert apart this morning. Once you have removed the material from the hooks around the top and sides (cut it off the hooks if the old piece is not to be reused) you can bend the insert to get access to the hog rings from the bottom and cut them with wire cutters… a lot easier than bending them as is suggested elsewhere.
Be careful because the plastic retainer that the hog rings hook into is not very thick. I broke it in one place and had to fix it using duct tape.
One of the videos suggests tightening the screws under the seat back foam… a couple of mine were loose and one fell out when I touched it.
The comments above have won me over to using zip ties to replace the hog rings. I came up with an alternative to passing the zip ties through the plastic like in the photo posted by ndfrigi… it looks like tightening the ties would be a challenge with that arrangement. What I did is to make a rod that goes across the plastic retainer and the zip ties wrap around it. I used a heavy duty coat hanger and bent hooks on both ends to prevent the ends digging into the plastic. A strip of duct tape below the rod hopefully adds a bit of resistance to the plastic and at least one layer on the top to insulate the rod from the metal springs in the seat back once it is installed.
In the package with my seat covers there was a piece of fabric backed foam 5/16 in thick and about 2 feet wide by 50 inches long. Is this just packing material or is it to recover the center of the bottom cushions? It looks like the same black foam in the middle of the cushion.