The basic way youi whistle is to make your middle finger and your thumb together into a loose oval. I have a small mouth, so I use the oval shape, but if it works, a circle is fine too.

Then you put the tip of your tongue against the place where your finger and thumb touch. You want there to be some pressure, but not too much. Just enough to bend your tonuge a little bit (as shown)

Lastly, you want to put your tongue and fingers back into your mouth JUST enough so that when you close your lips together the space your finger and thumb makes creates a small opening where your lips are not touching.

That is about it. The trick is to change everything your fingers and lips are doing untill you find a combination that works for you. Some things that are common to change:
--Shape of fingers. A cirlce will make less space open (because the extra room PAST your lips will be less or more, making a smaller or bigger opening between your lips
--Opening of Lips: you can pinch your lips together slightly to make a smaller opening. This is also how you make lower or higher notes.
--Pressure between tongue and finger. More will press more tongue closer to the mouth/lips making less of a gap for air, less pressure will make a bigger gap for air.
--Force of pressure: dont be afraid to really wail. The whole point is to blow HARD to make it really loud. I once lost some of my hearing from doing it as loud as I could in an enclosed space. Like a rock concert, loud. It was nuts.
Finally: blow, blow, blow. You WILL spend at least an hour, more like four, learning how. Just keep moving parts around anf blowing, untill you hear it go up or down in pitch a TINY little bit. When it feels strange, you're close. Just keep working with that till you get a tone.
Once you feel it, it'll feel just right. Like when you learned to ride a bike. Then you can start messing around with other fingers and pitches.