This can be done pretty much anywhere, but to be sure go to the map Downtown Los Angeles.
When spawning press triangle and look directly to the left, you should see a long decline with cars on it ( This is the same ramp you get the ungrindable trophy from).
Just balance your joystick and manny down the ramp and go left or right onto the next street and it should pop.
Make sure you do not bail off the manny because it will not count.