First, get in a motorcycle in Fuel Plant.
Then head north until you come to a wall at the end of the map.
Next drive into the building through the first big door.
Swerve around obstacles until you get to the stairs.
Drive up the stairs and on to the top wall (you will need to get alot of speed).
Then let your passenger get out (if you have one) if not, switch seats and get out.
You then should be on top of the building walking on nothing.
Once up there, you can kill your enemies without them seeing you!