How I did it: this is something I have wanted to do since I was very young, but never took the initiative to learn how until a couple days ago. I clicked the link from this site to learn how. It was very frustrating at first and I wanted to just give up, the first day I was trying and trying but not so much as a sqeek came out. I tried again the next day and out came some wierd noises and eventually a faint whistle. I kept at it trying different angles and finger combos and finally found the best for me is the ok shape using my thumb and middle finger. An extremely loud whistle is definetly a powerful tool.
Lessons & tips: try all different finger combos to see what works best for you, and don't give up even if it seems hopeless at first. look at the pictures, they give you a real good idea of the basic concepts......practice