A good way is to remove your spring, mount your largest tire, put it at full compression and give yourself something like an inch around the circumference of the tire. You may not be able to copy anyones exactly because if they have adjustable rear suspension their tire may be more or less centered in the wheel well than yours. When you get the first side cut, tape a large sheet of paper over the rear fender, trace the body lines and fender opening onto it, cut it out on those lines, tape it on the other side of the car and you will have the opening in the exact same place.