You should know that an open C tuning means that your guitar will be tuned in this patterns: C G C G C E. A standard tuning will make your guitar an E A D G B E; this tuning will totally change most of the things in your tone. Therefore, get ready to sound different. Moreover, try to start off by getting your guitar tuned to standard at first.
Start off with the last string. You have to tune it to E and since you know that a standard guitar would be E A D G B E, then changing the sound of your string will not be a problem. It should remain this way and you should remember that this string is the last string, which is also known as the thinnest string in the guitar.
The fifth string, which is known as a B string when the guitar is tuned at standard, it is considered to be a little thick as compared to the last string. You will have to raise its notes when you are trying to tune it to an open C tuning. It is supposed to sound like a C, so make sure that it matches the sound of the 3rd fret of the last string. Of course, the 3rd fret of the last string will be a higher C but you must make sure that the sounds match.
The fourth string, which is also considered as the G string in normal tuning, it should sound like a G when you are trying to make it an open C tuning. Since you are already set with the G string, you will not have to touch it and it should remain the same.
The third string, also known as the D string in standard tuning; it should sound like a C. Since you are trying to make it an Open C tuning, the third string should sound like a C. Therefore, use the 5th fret of the string underneath is so that you can tune your guitar to a C. However, the upper string is still an A and you can still use the 3rd fret to make it sound like a C. Tune your guitar accordingly.
Now tune the first and the third string together. The thicker E should sound like the C you just tuned.
Then tune the second string, also known as the A string in standard tuning, and turn it down to G. So now your guitar sounds like this C G C G C E.