The technique of whistling without using your fingers is that you need to hold the lips in desired position.
First, you must push the lower jaw forward slightly, the lower lip should fully cover the teeth. Try to make sure that it is firmly pressed to the teeth if you fail to do this the first time, you can help yourself with your fingers.
The position of the tongue is not necessary to strictly fix, it needs to respond to the flow of air, but the tip of the tongue should be moved away from the teeth for about 5-8 mm. When you exhale, air must first pass under the tongue and then through the space between your lips.
If you do not succeed the first time, don't despair. By training regularly, you will soon achieve the desired result.
There is also another technique of whistling without fingers, it differs from the first only by the position of the lips.

For starters stand front of a mirror and relax. Then squeeze your lips to form a small letter "O" if the air hole was very small.
Now put the language so that he was at a small distance from the lower teeth.
Start slowly exhale the air. If at first you do not get a clean sound, then you need to change the position of the tongue (maybe his back should be slightly lifted, and the tip of the tongue to push the teeth)