Pull back the corners of your lips so that they are stretching toward your ears. The upper teeth may show, but your bottom teeth should be completely covered.
Align your tongue in the center of your mouth. It should float in the middle of your mouth to create a ramp for the air to flow over. The tip of your tongue should be in line with the ridge of your bottom teeth.
Inhale into your diaphragm (the muscles in your stomach). Feel your sides and stomach expanding and push the air out forcefully using your abdominal muscles.
Direct the air under tongue and up through the sides of your mouth to create an effective whistle.