Because the Snark tells you what notes you are playing, it is very easy to tune your guitar down to a drop C.
In Standard tuning, the order of strings is as follows, starting with the low E (thick string) first.
From there, it's real easy to tune to drop C:
The Low E (thick string) drops down two notes to a C
The A string drops down one note to a G
The D string drops down one note to a C
The G string drops down one note to an F
The B string drops down one note to an A
The High E string drops down one note to a D
That's all there is to it. The thickest string drops two notes, all others drop one note.
If you're starting from a drop a drop D, and want to tune down to drop C, you simply drop all strings one note.
* You may notice on the video that I made a small screw up! While detuning the D string, I wasn't paying close attention, and tuned it to a C# instead of a C. OOPS!