Installing Rubber Floor

After patching and sealing the floor pans of our 66 valiant we are now ready to install the rubber floor, like the car originally had. The material I purchased was supposed to be molded, but that was about three years ago, and it is now very flat and very stiff. The papers that came with it suggest warming it for 72 hours before installation, and we are doing that now. Any suggestions on how to get it to conform to the shape of the floor especially in the back seat area? It is very thick stuff.