Align the mouth harp horizontally. Hold the larger, rounded end with your finger. Place the top and bottom bars of the thin end against your teeth, ensuring the thin rod in the middle has space to vibrate without hitting your teeth. Press the harp against your teeth with a small amount of pressure to help the vibrations to resonate and to keep the harp positioned in place.
Use your index finger to flick the knob at the end of the thin rod forward away from your face. If placed right, the metal rod should waver back and forth between your teeth, producing an audible warbling tone.
Flick the metal rod forward every few seconds as needed, and begin to move your mouth in different positions while keeping the harp still against your teeth. The movement of your mouth as you continue to pluck the harp should create different warbling tones.
Experiment with speed and style, and you become more comfortable with the instrument. The mouth harp can be a quick instrument to pick up, but can be painful for your teeth if misplayed. The more confident and relaxed you are in playing, the better sound and music you will produce.