what Luke_16 meant to say (ok, he said it, but obviously not clear enough) is that you hold the 7th fret on the E string. You play the A and E strings, then you retune the E string until it sounds like the A, all while you are holding down the 7th fret on the E string, so it is the E string you retune, the rest have supposedly already been tuned.
It also works to hold in the 5th fret on the A string and then retune the E string to sound the same, the only thing is that there there will be an octave difference between the notes you tuning, so it is harder to tell when they are perfectly in tune.
Hope that cleared a few things up.
