Instructions
Choose the fingers you wish to whistle with. A one-handed whistle takes two fingers, one being your thumb. To complete the whistle, choose either your index, middle or pinky finger. No finger is better than the other. The decision should be based on comfort.
Pull your lips over your teeth.
Make a U-shape with your fingers and insert them into your mouth up to the first knuckle. The tips of the fingers should be pointing at the tongue. The top side of the fingers should be pushing against the inside of the mouth.
Pull the tongue back, but do not curl it. The object is to flatten the tongue.
Take in a deep breath and forcefully blow it out. You should hear a faint whistling noise. Continue to practice. The more you practice, the louder your one-handed whistle will become.