there are 2 basic approaches, both involve removing everything that is there to get down to painted metal. after that, i would prime with self-etching primer (either brush on or spray on). there are many products, and i'm not going to dive down that rabbit hole, just talk about the basic concepts =
basic option 1 - there are many different spray on acoustic treatments, some can be brushed on. there are several thermal types of insulation that can be applied over the acoustic treatment - i am using dynaliner (comes in 3 thicknesses - 1/8", 1/4" and 1/2" and has an adhesive layer).
basic option 2 - put dynamat or dynamat Xtreme throughout the interior (floor, tranny tunnel, firewall, under / behind rear seats + doors). then cover with dynaliner
there is a 3rd option - to just put 1/2" dynacore everywhere as its sound and thermal - it also has an adhesive back. dynacore comes in sheets that are 1/2" and 1" thick
so what am i doing - since my car had just come out of paint, i didn't want to spray anything near the car, so i took the dynamat approach. then i learned that some of the liquid treatments could be brushed on (oh well, missed that one). i am then going to line the tranny tunnel with 1/4" dynaliner and then do the firewall with 1/2" dynaliner. i will probably do everything under the rear seat with dynaliner ... but we shall see. if i was removing / replacing the headliner, i would put 1/2" dynaliner there too. my car already has some thermal insulation on the inside of the door skin and if i make time, i might replace that as well. my recommendation, if you use dynamat, is to buy the 18" x 18" squares - its easier to deal with slightly smaller pieces than stuff that is 50" long.