Use a guitar tuner to tune your guitar to standard, E, A, D, G, B, E tuning.
Play the fourth string (the D string) of the guitar and let it ring out.
Play the sixth string (the low E string) of the guitar and let it ring out.
Turn the sixth string tuning key counterclockwise slowly, listening to the tone drop on that string. Watch your tuner; when the string registers a D note, you have it right. If you are tuning by ear, listen for the tone of the sixth string to match the tone of the fourth string.
Don't finger the sixth string when you play bar chords; let it ring out open. It will limit key signatures, but hardcore guitar is supposed to be simple and powerful.