While tuning each open (unfretted) string to a fretted lower-pitched string is the most commonly used method, it may give poor results. This is because pressing down on the string stretches it, making the note go slightly sharp. The effect is especially pronounced if you have light gauge strings and press down hard midway between two frets. Press lightly, just behind the fret, to minimize this sharping.
An alternate tuning method makes use of harmonics. Pluck the low E (6th) string while lightly touching it at the fifth fret. Take your finger off the string right after plucking it. This will produce an E two octaves higher than the open string. Now pluck the A (5th) string while lightly touching it at the seventh fret. You will hear the same E. Tune the A string so that the two harmonics are at the same pitch. Similarly, tune the D string so that its 5th fret harmonic is the same as the A string's 7th fret harmonic. Next, tune the G string so that its 5th fret harmonic is the same as the D string's 7th fret harmonic. Tune the open B string to the 7th fret harmonic of the low E string. Finally, tune the open high E string to the 5th fret harmonic of the low E string.
This method takes longer to describe than it does to perform. Once you try it a few times, you can tune very quickly and accurately.