How To Whistle - Mouth Whistle
1. Form your mouth into an "O" shape.
2. Make sure your tongue is touching the back of your bottom teeth. This is to ensure the air flow path from your lungs to your mouth is not restricted in any way.
3. Take a deep breath and hold it. Use your diaphram to push the air from your lungs, into your mouth, and out of the "O" shaped hole you formed with your lips. Continue blowing, using varying pressure of air, and contracting/expanding the hole between your lips to create a different tone or pitch.