Girls whistling will give people a frivolous feeling, not reserved enough, not mature enough, and will make people feel like that kind of casual woman. Of course, this is true in most occasions, especially in public. I suggest whistling in private.
Remember: the treble jet volume is low, and the bass volume is slow (jet: this is relative, not to let you blow hard, but to increase the airflow speed between your lips, and so is "slow")
Whistling method:
Method 1: when whistling, the mouth is O-shaped, the tip of the tongue is close to the oral skin, and air is blown out from the empty space in the middle of the O-shaped mouth. Try more by yourself, just master that discretion, and the most important thing is to practice!
Method 2: You can play the same sound without fingers. Press the tip of the tongue against the inside of the lower teeth, arch your tongue upward as far as possible, exert force on your cheeks, and close your lips. Anger, short and powerful.
Method 3: two fingers are O-shaped, the tip of the tongue is slightly backward, and exhale!