Step #1: Make an OK sign

Step #2: “Eat” your lips

Step #3: Cult your tongue back and put your fingers on the middle bottom of your tongue.

Push the air out of your mouth

The farther back your fingers are , the louder the sound is.
Next Don’t Read This If You Want Your Gun To Be Confiscated…
To People Who Want To Whistle With Fingers But Don’t Know How
All I got was a whole bunch of spit all over my fingers & two dogs looking at me curiously. Ah well, practice makes perfect. At least I can also whistle through my teeth 🙂