Get several reference photos of car designs you'd like your design to look like. For each design, take notes on what you like and don't like about the design. Also, circle or highlight parts you do and don't like.
Sketch an idea for your original car design on plane paper and pencil. Don't worry about drawing all details, but the more you can refine your idea at this stage, the less guesswork you'll have when you work with the design on the computer.
Use a digital camera or scanner to digitize your sketch. If you're using a camera, be sure to use a tripod to aid in producing a clear image. Also, do not use the camera's flash, which can wash out drawn images, especially those drawn in graphite, or pencil lead. Instead, use natural lighting, if possible, or use high wattage artificial lighting---about 100 Watts or greater.
Load your digitized design into a photo-processing program, then sharpen the image with the program's contrast and brightness tools. Optionally, apply image-enhancing filters such as those that remove speckles and noise. Consult your paint program's documentation for detailed instructions.
Open a graphics program that features vector-based tools. These programs simplify image editing by allowing you to shift, resize or scale an image's lines, rather than erasing and redrawing them.
Paste or import your scanned car design into the vector-drawing application. Lock the image so that stray changes don't affect it. Also, create a new, transparent drawing layer on which to begin drawing. This will allow you to trace over the scanned design.
Using your drawing application's curve-drawing tool, trace over each line of your design. Optionally, when you trace the wheels, use your application's ellipse or circle tool to accurately the wheel shapes.
Evaluate your design by hiding the scanned image of your design and studying the vector curves you've drawn.
Use your application's curve-editing tools to begin experimenting with changes to the design. Use the movement, scaling and rotation tools to adjust the curves you identified in the evaluation step.
Add color to the design by closing each of the vector curves you drew, then applying color to them. Consult the colors in the original reference photos as you work.