the hardest part is the window seal, it comes off with the door panel and needs to be reinstalled on the door before the panel is put back on, its just frustrating to get all the little tabs lined up and you feel like the panel is just on the edge of breaking the whole time you are removing it. its not really hard, just sketchy.
as far as sound damping, the best bet it to use strips or squares and add weight to large flat panels. you do not need to coat the whole door in it, you can, but its not required. if you have a limited quantity of material, things like the inside of the outer door skin are good places, because its almost flat and has very little or no reinforcement on the back, so you get a lot of buzzing and rattling. the goal of sound damping, at least the heavy rubber stuff that is most common, is simply to add weight to the panel, this lowers the panels natural harmonic frequency below what is audible, the panel still vibrates you just cant hear it any more.
2015 Chevy Silverado 2500 HD 6.6L LML