Well, you pucker your lips like you're about to give a person a peck on the cheek. Then open your mouth slightly to make a tight 'o'. It depends on where you put your tongue for what it will sound like. Then BLOW!!! If you hear a whistle, you'll know. Practice, practice, practice makes perfect!
Use your buccinator muscles to suck in your cheeks and then purse your lips as you exhale. Hopefully you will have a successful whistle.
"You just put your lips together and... blow," from the movie classic: "To Have and Have Not".
Crystal