Instructions
Load a level that requires you to use nitro. Levels that require you to ramp through a building or over a chasm typically require nitro.
Line up with the ramp. Once nitrous is activated, it will be difficult to steer the vehicle, so you want to be lined up well before activating it.
Press L1 when the "Special Action" marker appears on the top-left of the screen. Nitro will remain engaged until the action is completed.