Play each string of cello to the microphone, tighten or loosen the string to make the indicator on the screen blink and move to the corresponding string that you play.
if triangle is pointing to the left of string you play, that means you should tighten it.
if triangle is pointing to the right of string you play, that means you should loosen it.
The tuner reacts faster and more accurate if the microphone is placed closer to the string.
Touch the other strings with your fingers to prevent them from producing sound and disturbing the measurement.