Whistle Loudly with Your Fingers

How to whistle pretty loudly with your fingers. A great way to get someone's attention or just for fun.

Steps

Stick out your middle and index fingers.
Angle them so your fingertips of your middle fingers make a "v" shape if you were looking at them while they are in your mouth. Let fingertips have a minuscule gap between them.
Insert fingers in mouth against tongue. Put your fingers in just a little but past the first joint in your fingers. Don't push too hard against the tongue but let your tongue bend ever so slightly back from your fingers.
Make a "Sssssss" noise and guide your tongue to let the air flow through the little gap in your fingers. This takes a lot of practice, and most people don't get it easily.

Make sure your fingers are straight in your mouth. Don't have them angling down or up.
Make sure your tongue is angled right or else it won't work.
This does take some time to learn. Everyone is different.