Select a subject to draw. You will have three choices: draw from memory (or imagination), draw from a photograph, or draw from a live subject. Drawing from memory or imagination is convenient because this type of drawing can take place anywhere under virtually any conditions, and all that is needed is a piece of paper and a pencil. The downside to this method is that sometimes drawings made from memory or imagination are unrealistic, or lacking in realistic details.
Photographs are great to draw from as long as they are large enough and of a high-resolution. Drawing from a small photograph with poor resolution is often as bad as drawing from memory or imagination because it becomes difficult to create realistic details when you can't see them appropriately. However, if the photograph is the appropriate size and clear enough, it is an excellent resource for drawing from. Photographs are convenient because they are portable and unchanging.
Finally, drawing from real-life certainly does guarantee an abundance of clear details to aid the artist in his endeavors. Unfortunately, there are many downsides to drawing from real-life. For one, eventually the car will need to move and after that happens, replicating the conditions under which the artist began drawing the car will be virtually impossible. Second, unless the car is in a garage or some other covered environment, the movements of the sun over the course of the day will change the environment around the car, and the appearance of the car itself. Shadows will move, and as dusk approaches, the details of the car will become less clear. Thus, the artist who draws from real-life works with a natural timer and she must be confident in her ability to draw quickly and efficiently and accurately from the get-go.
Start with an outline. Draw the outline of the car lightly with a pencil. Ideally, these lines will be long, graceful and continuous. Sketching the car with short inter-connected lines will degrade the appearance of the final product, since cars are themselves graceful and smooth.
Once the outline has been made, draw inside the outline the larger pieces of the car. Again, these forms should be elegant and made with confident lines.
If you have trouble with confident lines, and you must make your initial lines sketchy and short, then your lines should also be made very lightly so that you can draw over them later with longer smooth lines more appropriate for the structure of the car.
During this time you should also create the basic structure of the environment around the car--do not wait until the car is completely drawn to start drawing what goes around the car.
Once the structure and basic elements of the car have been established, focus on the details. Tail lights, headlights, side mirrors, door handles and so on. Don't concentrate too heavily on the interior of the car--in fact, you may wish to leave out all details inside the car in order to focus on the glare on the windows or the windshield.
During this time you should also add details to the environment around the car, so that the entirety of the drawing is always drawn in equal stages.
The grill of the car, the texture of the tires, the appearance of lights and glare glinting off the metal or glass, all are things that should be added after details. Shadows also should be added at this step--start lightly and deepen the shadows gradually until they are the appropriate value.
Add textures and shadows to the environment around the car.
Darken your lines. With a practiced hand, reiterate the outline of the car and the most significant structural elements, like the lines of the door and windshield. Again, these lines should be graceful and long and for modern cars, very sleek.