Closing the lips to form an "O" shape is the first step in whistling. The tongue should be against the ridge of the mouth's roof, and the lips should be tight. Experiment with the mouth, but maintain the whistling position. Wet lips and blow slowly.
Top Tip Spot recommends that the tip of the tongue should be at the front of ridge of the roof of the mouth. Flatten all of the tongue until reaching the ridge. Position the tip of the tongue in such a way that is not quite contacting the roof, but still remains close. The lips should be tight, and the tongue should still be touching the roof of the mouth.
The tongue can also be positioned behind the bottom portion of the front teeth with a slight arch. If blowing using the bottom teeth, the tongue should be tightly pressed against the teeth.
Once the tongue is in proper position, blow as normal. A hole should be between the mouth's roof and tongue when whistling with the second tongue option. Whistling a song or tune is a good way to practice. There are two types of whistles one can make with the mouth, the Pucker Whistle and the Palate Whistle. It is also possible to whistle and hum at the same time.
Learn This notes that adjusting the jaw and tongue helps in attaining a strong whistle.