uh.. okay.. I know how to whistle like this.. I'd like to learn to do it without using two fingers..oh well..
ok basically, you fold your tongue..making its tip touch the middle of your tongue..when you're doing it right (in front of the mirror) you should see the bottom of your tongue.
Okay..with that done, you only have to put an index finger from each hand, above that folded piece of your tongue.. making it look like this:
/\ ..with the close ends on your folded tongue..lower your upper jaw on your fingers..then, simply blow air.Blow harder or softer..depending on how high you want your whistle to be.But in general, you have to blow a little bit hard to get the whistle out.
Try on your own..try pushing the fingers more into your mouth.. or less..until you get it right.
If you're finding it hard to do, try using two fingers from each hand (index + middle).. the sound its gonna make is going to be a little low pitched..keep practising until you get it right.