An electronic tuner , set to a chromatic scale makes tuning a harp a lot easier than tuning by piano or by ear and can be purchased relatively cheaply from your local store .
With most modern harps the strings are tuned diatonically starting at the red string (where the strings are of different colours) (red strings are C's), the string above that is D, then E, then F (blue strings are F's), and it follows the familiar do-re-mi scale, like the white keys on a piano. That will tune the harp in the key of C, when the levers are down. When the levers are up the strings rise generally by one note. If you have levers on your harp, they are used for setting the harp to different keys. For instance, if you set the levers on the F strings, the harp will be in the key of G; so if you begin on the G string (one string above the blue F string), you will hear the do-re-mi scale in the key of G.
Tuning a harp requires patience, dexterity and time and will need to be undertaken fairly frequently with a new harp, less so with an older and more used harp. If a harp is to be left unplayed for any significant length of time it is worthwhile loosening the strings a little before locking them in place using the keys.
Make yourself comfortable and do not wear any binding clothing, particularly at the shoulder. Balance the harp against your shoulder and pluck the first string, using the Tuning Key on the back of the tuning peg either tighten or loosen the peg until the correct note is achieved. Leave the harp after the initial tuning and retune in an hour again. The length of time the harp remains in tune lengthens as it is used.