Took me 2 years to figure out how to do it. 
All the guides on the internet are useless. (Click on the 4 steps button. It's not just the Karate Chop. Last step didn't work for me
)
http://www.instructables.com/id/How-to-Whistle-for-those-who-CAN-NOT-Whistle/
I just blew air like a bomb falling until a whistle-ish sound came out 2/3rd's of the way down the register. (Or you can go up like jet engine revving up)
Then I focused on that for a few days and expanded my range from that proto-whistle.
That hugely loud piercing whistle that can stop traffic using 2 fingers?
Still never figured out how to do it.
Anyone still struggling with whistling?
Never learned how?
Maybe this thread will help you to whistle "Pop Goes the Weasel" like Data on the 1st episode of Star Trek.

All the guides on the internet are useless. (Click on the 4 steps button. It's not just the Karate Chop. Last step didn't work for me

http://www.instructables.com/id/How-to-Whistle-for-those-who-CAN-NOT-Whistle/
I just blew air like a bomb falling until a whistle-ish sound came out 2/3rd's of the way down the register. (Or you can go up like jet engine revving up)
Then I focused on that for a few days and expanded my range from that proto-whistle.
That hugely loud piercing whistle that can stop traffic using 2 fingers?
Still never figured out how to do it.

Anyone still struggling with whistling?
Never learned how?
Maybe this thread will help you to whistle "Pop Goes the Weasel" like Data on the 1st episode of Star Trek.