I am working on the same project right now for my daughter's house. Here is a tutorial that I referenced.
https://weallsew.com/upholstered-cushion-cover/ I have made window seat cushions before and purchased the green foam from JF. That is easy to cut to size with an electric kitchen knife. I am using home dec fabric for this cushion, but I have used quilting fabric before that I quilted using a grid. I have made them with piping and without, mostly a personal preference for the look you want.