Are they door speakers? If so that's where I'd start. Take off the inside of the door and you put the Dynamat on the inside of the metal of the door. Look around for posts about it and you'll find plenty of pictures of how people have done it.
I wouldn't buy Dynamat anyway...there are others, like RAAMmat, that do the same job for cheaper.
As for deadening, I would start on the doors like previously mentioned - then if you have a trunked car, the entire trunk usually is a good place for more.