Wash and dry your hands
Make a circle with your thumb and index finger, or use both index fingers
Place the tip of your tongue against the roof of your mouth
Open your lips slightly
Put the finger shape into your mouth until it touches your tongue
Fold your tongue back a little to create a small air channel
Blow air steadily through the gap
Adjust finger position, tongue placement, and lip shape until a whistle sounds
Practice with small changes in angle and air pressure
Keep trying different finger combinations if one does not work