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Make sure that the display shows the note that the string is playing and a needle that moves to the left or right of its meter display. The screen will also typically display yellow when the note is too low, and red when it's too high. The combination of colors helps you guide your tuning until the needle stabilizes into the center of the display with the desired note above it.  To tune to G, the notes—from bottom right to top right when the narrow end is facing upward—should be: G, A, B, C, D, E, F, G, A, B, C, D, E, F, G. Alternative tunings are different, but stick to G standard tuning until you get more experience.  Many lap harps come with a sheet with all the notes marked, as well as tuning instructions. Be sure to have something of this sort (or write down the notes above on a paper) so that you have a  reference for what notes each string needs to be tuned to. For example, G♯ (G-sharp) is a half step up from G, and G♭(G-flat) is a half step down from it. Most lap harps are tuned to the key of C or G and only play "natural" notes (no flats or sharps). If you're an experienced musician, don't be afraid to play around with some of the strings to turn a note or two flat or sharp. If the note is too low—for example, the first string is A instead of G—you need to tighten the string a full step. Hook the tuning key onto the silver knob, strum the note clearly until it shows up on the tuner, and then slowly tighten the string with your tuning key by turning it counterclockwise.  As the note is fading away, you'll hear the pitch begin to change. Use this as a guide as to how much you've changed the sound, and if you're going higher or lower. Keep an eye on the tuner screen and stop tightening the string when the tuning needle centers. If a note has gone sharp—for example, the bottom-right string is A instead of G—hook the tuning key onto the string's peg, strum the note clearly, and gently turn it counterclockwise. Stop loosening the string once the tuner reads G.  Watch the screen carefully and stop turning the key once the tuner displays the correct note. Keep an eye on the tuner screen and stop tightening the string when the tuning needle centers. Tuning can be slow and tedious work, so take plenty of breaks. If you come across a particularly difficult string, come back to it later. Check your progress periodically by playing up and down through the strings you've tuned. Make any adjustments as you go along.
Strum each string and observe the note on the tuner. Note how many half or whole steps the string is away from the intended note. Tighten each flat string by slowly turning the tuning key clockwise. Loosen each sharp string by turning the tuning key counterclockwise. Continue this process with all the strings until they're tuned.