Not hard at all - just take your time doing it. I just put in door and trunk seals recently on my Falcon. Peel/scrape off the old weatherstrip, clean up the remaining gunk, and glue the new weatherstrip in place with adhesive you can buy at auto parts stores. You put the adhesive in both the channel in the door or body and on the weatherstrip, and you'll have a lot easier time if you put the glue on the weatherstrip as you go along rather than doing the whole thing at once and getting adhesive all over yourself and the car.