Wet your lips slightly
Purse your lips into a small round opening
Keep your tongue low and slightly back
Blow a steady stream of air, not too hard
Adjust your lip shape until a clear whistle forms
Tighten the opening a little for a louder sound
Use your fingers if needed to create a stronger whistle
Practice changing tongue position for volume and pitch
Blow with control instead of force
Repeat until you find the loudest position for your mouth