It depends- may be easy, may be a major PITA. If it has internal cording, springs, etc, probably wanna leave it to a pro. If it is a typical office chair, it is a plywood/plastic/metal shell, a hunk of foam, and fabric stapled or glued over it. Hard part will be figuring out how to get it apart w/o trashing it. Local upholstery company/boat cushion/tarp company will have the fabric and the foam, or possibly even a big sewing supply store. Get a slab of <high-density> foam and carve to fit, use dabs of glue rated for foam to keep it in proper place on base, and then strech the new fabric over it and secure it however it secures. If old fabric had sewn shape, or was 'molded to shape', you will have to figure out an eye-pleasing way to fold and tuck everything so it looks right when reassembled. Fabric store will have the buttons and cord for any buttons- I usually ignore such details. Check library or big-box DIY book aisle- I'm sure somebody has a book with pictures.