It's not that hard as long as all of the little holes in the body for mounting the weather strip are still in good condition. Just take your time. On my convertible I used some black RTV (silicone) on the windshield posts to help keep the weather strip where I wanted it. Just tape it in place for 24 hrs. with masking tape if you use the RTV. Also, you might find it hard to close the doors and trunk after the new stuff is installed, just give it some time, it will compress where it needs to.
Which weatherstripping? The hard top roof rail weatherstrip is going to take some effort to do correctly.
I assume you are talking about JUST the ( Roofrail) upper door/window weatherstripping...Clean the residual adhesive and rubber from the track. Use wooden popsicle sticks to guide the edges into the groove/track. (See reference) . I've used Soffseal products several times with good results. Here's some more guides: https://www.carid.com/images/soffseal/pdf/installation-guide-7.pdf Use 3M black adhesive VERY sparingly AFTER trial fitting Buy a small tube at Walmart (1OZ is enough!) ; you'll be surprised at how little you actually need! The trunk only NEEDS a 'dot' on the ends; likewise for the lower door weatherstrip. Now if you are talking about the window "Fuzzies", the body part is simple, after you have removed the door panel. The door panel half of the "Fuzzies" is a whole 'nother animal. I tried to do them on a Regal years ago... Key word was"TRIED".
The issue with installing the hardtop roof rails is that all 4 pieces of glass are going to need to be readjusted. Plus the fact that you need to access inside the sail panel to get the old rubber out and the new stuff installed properly. So door panels off, rear seat out, inner quarter panels and armrests and roof rail windlace all needs to come off. I did it on the beater. Something I thought was going to be a slam dunk turned into an all weekend deal. But the results were worth it
Soft seal video must watch. Apply glue to weatherstrip. Apply glue to were it’s goiing. Touch the (2) together. Pull apart. wait 10- 15 min. That process is everything.
Aftermarket roof rail strip sucks. It’s much harder than original which was foam with latex skin. No one knows how to make it correctly anymore. I think it was super toxic to produce. Thank the greenies for another one. Another reason I sold my cars.