It is essential that you are able to tune the guitar to itself, meaning it is important to be able to check each string with the other strings on the guitar. The following method assumes the guitar is already tuned in standard tuning.
Place a left hand finger on the fifth string at the 7th fret (a B note), and play the string. Play the open fifth string (an A note). Turn the sixth string tuning key slowly in a clockwise direction, lowering the pitch of this string until the note on the 7th fret is the same pitch as the open fifth string. The note on the 7th fret will now be an A note and the open sixth string will be a low D note.
Place a left hand finger on the sixth string at the 12th fret (an E note), and play the string. Play the open fourth string (a D note). Turn the sixth string tuning key slowly in a clockwise direction, lowering the pitch of this string until the note on the 12th fret is the same pitch as the open fourth string. The note on the 12th fret will now be a D note and the open sixth string will be a low D note.