To get a whistle out of your mouth, you must first form an “o” with your lips and blow air through it.
Keep blowing until the sound becomes louder and more shrill. Then, quickly place your top teeth on the tip of your tongue, and use either hand to cup around your mouth to form a sound chamber.
Quickly bite down hard onto the lower part of the whistler’s tongue (be careful not to bite your own!)
And finally, blow air through the resulting gap in the teeth. The greater the force of the blowing, the higher the whistle will be pitched.
So this is how you can whistle.