Teach me to whistle loudly with my fingers.

So I’m sitting here looking at instructions on how to do the loud taxi-hail-style whistle
with the index finger and thumb of one hand, but I’m having a bit of trouble. I can’t visualize what aspects of my mouth and fingers create what part of the whistle, so I don’t know what to change them to get it right. Here are the instructions I’m using for reference. Anyone have any hints or tips?