It's a pretty easy job. The covers have J-channel at the bottoms that snaps onto the bottom of the seat frame. If you are using dealer OEM pre-shaped foam and seat covers, it can be costly. Just the driver side butt foam is almost $100 from Ford. I believe the seat cover costs even more. The seat back cover is easier to slide on if you put a plastic bag or thin sheet of plastic over the foam. My '97 had plastic there from the factory. You can buy foam and foam glue from an automotive upolstery supply shop.
I need to stiffen up the butt pad on my '97. I'm trying to figure out if I can just glue some extra foam on or under my existing foam, before installing my new take off seat covers from E-bay ($125 for the front seat).