First make sure the area is clean and the is absolutly no water. Then just spray it on. Make sure you get the paintable kind because the other type doesn't dry completely.
I've found this is better- Go go Home Depot and buy some roofing felt. Spray 3M adhesive on the back and you've got Dynamat! It's not pretty but it works. You can do the whole truck for the price of one piece of Dynamat. Then you can get polyfill, aka pillow stuffing, and pack all empty areas. A huge bag of this stuff costs 2.50 at WalMart.
I'm done doing anything to my truck for now, the snow is falling. In spring I'm going to totally sound deaden it.