Tune all six strings as fast as you can, starting with the thick low E. Each string locks in when it's right, then the next one lights up. The clock starts when you pluck your first string.
Nothing is recorded or sent anywhere. Your device listens through its microphone and throws the sound away as it goes.
Tip: pluck one string at a time, let it ring, and turn the peg slowly. A quiet spot works best.
Playing the wrong string? The page will tell you which one it wants. A string can drift while you tune its neighbours, so it's worth a final strum of all six.