If your mat is already damaged, center the MatRx over the damaged area. The damaged area indicates where your heel rests.
If you have new mats, remove the backing and put it on again exposing a sliver of adhesive. This will hold MatRx in place while you test placement without using full adhesion. Test by pivoting your foot in your normal driving position and adjust as normal. If fitment is correct, you should not feel the product at all beneath your heel.