The C String (the fattest string – #6)
To get the C string you need a reference tone. You can use any C on the guitar. Luckily there is one on the 5th string (second fattest string). Fret the 3rd fret on the 5th String and lower the 6th String so that the pitch matches. Because you are lowering it 4 semi-tones you will have to turn it at least one full turn on most guitars if not more.
The G String (the second fattest string – #5)
Okay to get the 5th string you will need to use a reference tone that is a G. Since we started in standard tuning the 3rd string is a G so you can use that. This time you are going to lower 1 full step. Pluck these 2 open strings (5th and 3rd) and loosen the 5th until the tone matches.
Another C String (the third fattest string – #4)
Since this is a C just like your fattest string you can just pluck those 2 strings together and adjust this 4th string until it matches the pitch of the fattest string.
Another G String (4th fattest string – #3)
Guess what, don’t do anything, it’s already a G since we started in standard tuning.
Another C String (5th fattest string – #2)
This one you will need to raise up. Pluck it together with the 3rd or 6th fattest strings. Since they are all C’s all their tones will match, though they will be in different octaves so it will sound kind of like an echo. This string you need to tighten, just a little bit though probably about a quater turn on most guitars.
The E String (The skinniest string there is – #1)
Leave it as it is; your all done!
Source :http://www.howtotuneaguitar.org/tuning/open-c/
