Set all of the controls to the middle of their range, usually at 12 o'clock or "5." This will allow you to hear the effects of your changes.
Slowly increase or decrease the bass control until you have the desired amount of low end power. Heavy metal distortion should have plenty of bass drive, but don't let it get muddy and undefined.
Bring up the treble control to put definition in contrast with the bass. Don't let your tone get harsh, but make sure it won't be lost in a mire of low end. You may need to sweep the bass and treble controls at the same time to find a balance.
Adjust the mid control to your needs and preference. Cutting the mids will make that "scooped" cornerstone rhythm sound, whereas boosting will give more of a harmonically-rich sound to leads, or a crunch to rhythms. Maybe knock off just a little bit to smooth out your favorite lead and make it blend better with the song. Take your time here--the middle ranges are the most important to distortion tones, so use your ear and find what works.