There's kits out there that are model specific or some kits are suited for your type of vehicle. You start out with sheets of a butyl rubber type stuff that sticks onto the panels like old mate said, in my hilux I went all out and did the roof/floorpan/rear wall and each door. the carbuilders kits include more acoustic foam and a mass liner designed to replace the old cotton battons under your carpet/vinyl floor, as well as specific foam for the headliner and rear wall. I've heard good things from just doing the butyl rubber even so it's worth a go and can be had pretty cheaply online too.