Your tank is glass, yes? If so:
First lay newspaper at the bottom of the tank. Now cover the paper with aspen wood shavings, I usually lay the bedding about 2/3 inches deep. As for the heat mat, your heat mat should go underneath the tank, and cover around 1/3 to 1/2 of the tank. It's best to raise the tank slightly off the heat mat so there's some air circulation. Your heat mat should be connected to a thermostat to stop your tank overheating.
I no longer use glass tanks, but that's what I used to do and it worked very well.