Don't use one. I bought a new board last month and completely forgot to get a stomp pad before a weekend trip to Vail. Rode both days without one and realized it doesn't really make that big of a difference.
That being said, when I did use one I had it positioned right next to the back binding and pushed my rear foot into the back binding for extra support. Now without the stomp pad I just push my rear foot into the back binding.