The seat back cushion should be split down the sides of the of the middle section but left together at the top. The center is then tucked into a pocket on the back of the center section of the cover. In the ones I did years ago there were a series of hooks on the 2 vertical rails of the seat back frame that attached to the center section of the cover to pull it against the frame. The bottom tucks under the bottom of the frams and is held by a metal strip. There’s a bit more to it than that but it’s been so long I don’t recall the whole process.