Taught myself back in my teen years, for me its both index fingers in to the first knuckle touching my tongue that is curled down to my lower gum.
Loud enough to hurt my own ears.
It's come in handy a few times, used it to locate hiking buddies in the distance or get a dog's attention. Very similar in tone to a Marmot's whistle, enough that I've got them to spin around and take a good look at me, then again maybe they don't like it
