Remember being a kid and everyone said that tracing was bad. Well, all industrial designers do it. So, take some pictures of what it is you will be creating or changing. A few angles of your car that would look good modified or whatever. Then enlarge it to the size you want and get some tracing paper. Then lightly trace the car adding the things on that you want. Once it is all lightly traced and drawn, then you can go over it with pens to darken all the lines. You will darken the lines that hide a plane behind it and that are the outside plane twice as much. Hope this makes sense. It is just a line weight thing that makes quick drawings stand out more. Once you have your line, pen drawings I would scan and copy them. These you can try your hand at coloring by marker, or pencil. If you know illustrator you can scan them into the computer and color them that way. Brake anything down to steps and it is easy right? Please ask any questions if you want, good luck