from standard, drop your low E string to a low D, do this by fretting the low E string on the 7th fret, and make it sound the same as your open A string, once they sound the same, your tuning is DADGBE. now, if you know how to tune your guitar in standard tuning, by fretting the 5th fret on the low, now D, string, then making your open A string sound the same as that, and continue like that, until you get to your B string, in which case make your B string sound the same as your G string( should be an F by the time you're there ) fretted at the 4th fret, etc...then your tuning should be DGCFAD, now simply fret your low D string at the 7th fret, and tune it down until it sounds like your open G string, now you're in drop C, CGCFAD, and you can continue the same formula to tune this like a standard tuning, to get to standard C, which would be CFBbEbGC...very confusing i'm sure.