Your left index and middle finger will rest in the space between your right thumb and fingers. Your palms will meet to seal off the back of your hands. Your right fingers will rest on top of your left fingers.
Take a deep breath and then exhale slowly and evenly into your hands. It should take 10-12 seconds to run out of air.
There are, of course, limits to this, as opening too much will prevent any whistling at all.
