I used Peel & Seal and you want to make sure you have a nice clean surface. I cut mine in big pieces, but it really doesn't matter much as long as it sticks.
I used two layers on the floorboards and the firewall. I also used a wire wheel to clean off some of the crap on the wheelhouses behind the rear panels. Then I used tack rags for painting to get as much loose dust out. The problem with my car was that the interior sheetmetal was primered and yours may not be such a pain.
This shows how mine looked when I got started.