The oil drain plug on a 2000 Ford Mustang is located on the bottom of the oil pan. To be more specific, it's on the front-most part of the oil pan's underside. You'll need to get underneath the car to access it.
Because there are different engines available for a 2000 Mustang (3.8L V6 and 4.6L V8), the exact position might vary slightly, but it will always be in that general area. A shop manual specific to your engine would provide a precise illustration.
