A lot of heavy metal and alternative rock musicians tune their guitars down to create a darker, deeper sound. Guitarists commonly tune their instruments in c-tuning, which looks a lot like standard e-tuning only a few notes down per string.
If you haven’t already, make sure your guitar is in perfect standard tuning. That means the fattest string on the bottom should be tuned to low E, and the second fattest string A, and the third fattest D, and the fourth G, and the fifth B, and the top string a high E, one octave above the lowest string. To make sure their tuned, hold down the bottom E string on the fifth fret and it should sound identical to the A string. Hold down the A string on the fifth fret to make sure it sounds like the D string. Hold down the D string on the fifth fret to make sure it sounds like the G string. Okay, now hold down the G string on the third fret to make sure it sounds like the B string. Hold down the B string on the fifth fret to make sure it sounds like the top E string.
Now, with the aid of a keyboard, you can tune the top and bottom E strings down to C.(Tuning each string a little lower and then up to where you want it will prevent the string from getting out of tune too quickly.) Now, adjust each string to accommodate the top and bottom strings. Tune them the same way you did in step one, so the strings from lowest pitch to highest will be C-F-B flat-E flat-G-C.
Now your guitar will sound just like your E-tuned guitar only a few notes down. Have fun!(If you miss E-tuning, you can always readjust the strings).
Tips: Try playing familiar songs that you would usually play in standard tuning in c-tuning. The song will sound very different and, probably, darker and more depressing.