First under the seat it has some 1.5 X 1.5 pine timbers making the seat frame. I doubled it against the backing by putting another one directly under the existing frame. Screw some #8 2" screws in them.
Then in the new back I made a 48" X 32" use the old back as a template and cut it to the same angle to fit the cushion. Then I
add some 1" X 1.5" and glue them to the back. So this I attached to the wall and long one on the floor.
Screwing under frame into the back with the 2" screws, several of them. Since it has a nice one inch top I left that old silly topper made from week cardboard off.