I did some both ways. Wedging a piece of wood or whatever between the plastic panel and the door gives just enough room to get an arm or two up inside to put on a pre-cut dampener sheet and roll it out some. I used a 1' dowel piece to press hard to reach areas down. Could use whatever you have on hand. Wear long sleeves to avoid getting scratched up.
It was much easier with the window unbolted, much more room to move. Either way, the work is worthwhile in my opinion, bass tightens up nicely and cleans up the music. I like the sound of the door closing too.
I was shocked by the lack of factory dampening on the inner door skin. One short swipe of a brush on deadener. No wonder the door sounded like an old clunker before treatment. Pictures are before, driver's door and after kilsmat.