How to do drag whistle in beatbox?

Get the tip of your tongue touch the back of the bottom row of your bottom teeth
Have the middle section of your tongue touch the alveolar ridge (the place between your upper teeth and the roof of your mouth)
Make sure there is some space in the back of your mouth and towards your throat
Breathe in through a tiny gap between your tongue that's against the roof of your mouth
At first, it'll come faintly, so practice and continue building that pressure and control