If you want to start with a basic application, hit the doors first. Remove the door panel and layer inside the skin of the door as well as around the mounting hole of the speaker, as well as that entire plane if you have enough. The second most important area to get to is the floor (not the roof as so many people think). Contrary to popular belief, sound deadening is not to take away rattles, although it sometimes helps. It is to keep road noise out and your music in. It enhances your speakers capacity to produce sound by "deadening" the surface it is mounted to. Think of your speaker as a trampoline and you are the air it moves (just follow along). Are you going to be able to jump higher if the tramp is on a solid surface, or one that moves along with it?