Learn how to whistle very loud with your fingers inside your mouth in simple steps. 1. First fold the front part of your tongue backwards and insert any two fingers from both hands, then blow the air out. 2. You can use your two small fingers, index fingers, middle finger and; others to blow out the whistle. 3. The most difficult part is using your thumb and index finger together to blow a whistle. 4. You can also use any two fingers together and also your thumbs by practicing with them to perfect.