Whistling Tom explains that the first step in whistling with your mouth is to put the tip of your tongue against the roof of your mouth. The air that flows between the roof of your mouth and the top of your tongue is what dictates the note that leaves your lips.
Next, say "eee" and feel the sides of the top of your tongue touch the inner edges of your top molars. The top of your tongue should be close to but not touching the roof of your mouth. However, the tip of your tongue should curve downward so that it is behind and parallel to your top front teeth. Your tongue should never touch your front teeth. Then, with your lips held in a tight pursed position, say "poo." With your lips held in a round shape say, "pah, tuka, dada, ladle, loo, oodle, you-you-you, oy-oy-oy, oy-you," while observing your tongue and lips. All of the previous mouth movements are used when whistling, according to Whistling Tom. Next, softly blow air through your puckered lips using your tongue to direct the air in or out. Continue to do this until you are able to make the sound of a single note.