1) Start up the mission "Run For Your Life - Shangri-La Drugbust".
2) Pass through the fields of pot.
3) You will see two police men.
4) Sneak past them both.
5) Then, turn left and then right, and go into the back door.
6) Go over to the DJ radio.
7) Turn it on.
8) Crouch behind the desk and angle the camera to the dance floor.
9) Look at the police men and they will be dancing and smoking pot.

Here is a video tutorial: