I did this a couple weeks ago. I also put some RTV clear silicone around the outside edge of the new gasket where it meets the plastic, and on the seam where the ends of the new gasket meet.
He didn't clean the glue from the old gasket that well. It took me a long time, but I got almost 100% of the glue off by letting goo gone soak on it several minutes. I dabbed it on with a cloth (if you spray be careful not to get it inside housing openings). Then I heated up the glue and scraped with a blue plastic trim removal tool I had from Crutchfield.
Used middle one in pic.
Set of 3 pry tools helps remove interior and exterior trim and door panels
www.crutchfield.com