Insert the jack from the mic clip into the tuner and set the tuner on the floor, a chair or a music stand. Don't hold the tuner as it will slow you down when tuning.
Place the mic clip on upper portion of the T-brace on the harp. Anywhere on the T-brace is fine, but it is most commonly placed about one foot from the top of the harp. The T-brace is the rounded portion of the harp that is opposite the performer when playing.
Find the A4 string. This is usually the 6th string from the inside of the harp. Your harp may be tuned slightly differently; if that is the case, just locate your A string that plays in the middle of the treble clef staff.
Pluck the string and look to your tuner to see if the string is out of tune. The tuner will tell you if your string is sharp or flat.
Attach the tuning key to the tuning peg that corresponds to the A4 string. Tighten the string by turning the key to the right if the string is sharp, turn it to the left to loosen the string if it is sharp.
Check the string again by plucking it to see if you have achieved the correct pitch. Continue to adjust the A4 string as necessary to tune the harp correctly.