whistle loud with two fingers in my mouth

Walking to school I thought it would be fun to learn to whistle with my thumb knuckles…like old movies. I could not figure that one out.
A friend could put the index and middle fingers in his mouth and blow….Loud whistle…It could be done.
And then is was trial and error. Where is the tongue? Put the four finger tips on the front curve of the tongue. Push into the mouth …a little and blow. When you find the right…push…the right spot for your tongue it will be a whistle. And soon you will automatically go to that spot, and learn tunes, and volume.
...signal friends, whistle at ball games…have fun and even embarrass yourself once in a while.

I want to whistle loud with two fingers. Perfect for concerts or calling my dog.