I don't know of any tutorials, but I can give you a quick and dirty that usually yields nice results with some tweeking. First off, create a poly plane and scale/position it above the car so that it is about the same size as the car. Assign it a new lambert and set the Color and Ambient Color to full white. Now create a light (any type of light will do) and turn its intensity to 0. Select mental ray as your renderer and in the Render Globals under the mental ray tab change the Quality Presets to Preview Final Gather. Now render one out and you should have a fairly nice looking test shot. To tweek the settings, play with the FG Rays and Min/Max radius settings in the Final Gather tab. Hope this helps.