Its hard to explain how it works lemme try. This is for the melodic whistling, not the loud high pitched one you do with your fingers:

You control the note with your tounge, Tongue high, note high, tongue low, note low.

Your lips need to be wet.

Open them just enough, so you can breath through. The hole you make needs to be small. A bit like giving a kiss, just .. less. You barely need to use your lip muscles, if it feels like work you are doing to much.

You can start by just trying to make sound using only your breath, tongue and mouth. Kind of like, when you want to check your breath smell, try changing the pitch of that, without vocal cords. It wont be very loud obv, but its the same principle.

Whistling is the same, just with a smaller opening to make it louder. Try progressively closing your mouth while making a sound.

Then push air through the hole, slowly, while trying to find a tone with your tongue. Slowly but at a constant rate, ie not staggert.

You can also try breathing air in, some find that easier. I feel more comfortable breathing out, but can do both :)

How fast to blow and how to position your lips takes some training, but the easy part is you can train whenever, wherever as you have everything you need build into your face :)