Position the tip of your tongue so that it almost touches the roof of your mouth. The tip of your tongue needs to bend slightly downwards and the flat part of the tongue must come close to touching the roof of your mouth. The tip of your tongue should line up with your bottom teeth.
Make a "woo" sound with your mouth and hold that position. You should notice that your lips create a circular hole in the center. This shape is crucial for creating a whistle.
Blow air through your lips until you can create a successful whistle.
Increase the pitch of the whistle to create a high-frequency whistle by bringing the tongue further back into your mouth and increasing the tension of the lips.