You can start off by finding one of the notes. C# for example.
C# is located at 2nd fret B string (the second thinnest one).
Ok so now you know that your 1st and 6th string should sound like that.
Since both 1st and 6th strings are E, and C# is just benieth E, tune down.
Then find another note in the tuning, G# for example, and do the above with the string supposed to be G#.
Things may get confusing when you've tuned all of the strings, since you may
not know what notes are where anymore.
Well, just count!
You know that your 1st string is C#, so to get to G# just go one fret at a time:
C#, D, D#, E, F, F#, G, G# (E and F are right next to eachother, so there's no E#)
So G# would be placed on the 7th fret on a string tuned to C#.
Then you can just repeat these steps to get the tuning you want.
As long as you know which strings are tuned to what note, you'll be ok.
If you don't know where the notes are to begin with, there are alot of fretboard
maps and such on the internet, and probably even on this site.