How to do common whistle in beatbox?

The common whistle is the typical whistle you hear used in everyday life, which is created by breathing a focused stream of air through a circular gap between your lips.

How it's done
1) Make sure your lips are moist
2) Say the word "who" and leave your lips in that position with a circular gap
3) Blow out a steady stream of air through that gap.
4) Try to think of a certain note in your head and replicate it!