Example: Tuning a low F string
Play the nearest F string that you know is in tune. Then quickly pluck the string you want to tune and listen to both strings together. If the low F string is out of tune, this will sound most unpleasant. The trick here is to turn the low F’s tuning pin by tiny amounts until both strings sound in tune. Hearing in this way takes practice and most ears can be trained: practice tuning by ear with strings in the middle in your harp, as these are the easiest to hear, and check your tuning with your tuner.