Couple Things I Learned Doin' 1 last week.
Don't know if you have taken the old one out yet, but when you do, there is an amazing amount of "Crud" that has somehow got in there and hardened. Something to scrape the tough spots and strong hose stream gets enough of it out. A little warmth is helpful to make the rubber a little more pliable. I started feeding the top edge in with a turnbuckle wrench on a folding riggers knife 'till I added it to the tool collection under my dock. Then used a 2 inch plastic putty knife or scraper with the edge dulled. Still took me about 2 hours, not too hard, just tedious. Rubber or non-marking hammer might be useful, once in a while you need to whack it (like with heel of hand) to seat it in the groove.