I have vivid memories of learning to whistle when I was about 5 years old. I walked about for days on end with my lips puckered, blowing like mad, until the day came when a soft whistle emerged and I was soon whistling like crazy. Some years later I started to practise whistling with 2 fingers in my mouth. It took time but I got there.
There's only one answer - persevere. As Lauren Bacall said, "You know how to whistle, don't you Steve? You just put your lips together and blow".
try putting your 2 middle fingers and your 2 fore fingers together to create a downwards V shape then with your fingers push the tip of your tounge slightly upwards and blow if nothing happens keep trying just very slightly shifting the position of the fingers and tounge and from there practice at every opportunity and one day you will get it.
I haven't whistled for years, so I've just had a crack at it.... I can still do it with and without fingers in the mouth.. Follow Mummiboo's instructions and keep practising..