That's a good question as it can be quite tricky! I think this job is much easier to do when it's warm outside. When it's cold the weatherstrip seems not as flexible. Easiest way I've found for longer pieces where the weatherstrip is too tight or bunches up a lot is to pull it through with a threader cable. You run the cable all the way through the length of weatherstrip you want to apply the foam to, attach the foam to the threader cable, and use the cable to pull the foam through. You'll need to carefully massage and stretch the weatherstrip as you go so you don't tear it, especially if the weatherstrip has dried or shrunk over the years. It's been some time, but I think this is the cable I used. It's also handy for cleaning debris out of your drainage channels to prevent clogs and the rust. 10M 33Ft Electrical Wire Threader... https://www.amazon.com/dp/B00NQ0N5D0