If you're reusing your old foam steam the foam first, it will refluff the old foam add batting/flecing (buy at fabric store)to add even more bulk. On the old covers there's a wire that hog rings the center cushions bases and backs to the frames remove it and reuse it on the new it not easy to reinstall but can be worked in.
Set the new covers in the sun and let them get nice and warm. Hog ring the centers down about ever 3" inches or so and add one in the corners and again set in sun and let warm up again. Before pulling the outer edges over install all screws in to holes such as hinge covers and back stops this will make it easier to find the holes after covers are pulled over.
If you got the centers hog ring down proper, start pulling the edges over and around the frame starting in the front and working backwards, hog ring equaly left and right so when you to the back the cover is center on the foam, finish the back pulling tight. Locate all screws and cut a X on the tops remove screws.
Wrinkles if using old foam is a way life but steaming first will make the old foam come alive and most will come out if you pull the covers and hog ring proper. The hardest part is pulling the backs over the hinge pins