Wet your lips slightly
Pucker your lips as if saying “oo”
Keep your tongue relaxed and slightly back
Leave a small opening between your lips
Blow a steady stream of air through the opening
Adjust the lip shape until a whistle sound appears
Move your tongue forward or back to change the pitch
Tighten or relax your lips to make the whistle louder or softer
Practice with short bursts of air
Try different mouth shapes until it works