I start out with an 8.5x11" piece of cover stock weight (takes marker well and holds up to abuse over time) recycled style paper. I buy this in large packages at the office paper supply store (much cheaper than getting a similar paper at an art store). The paper here is Neneh brand Desert Storm. I start out with an erasable blue or regular lead pencil and quickly put the idea down on paper, simultaneously considering the product I am designing, the purpose, it's buyer, current trends, future trends, styling theme, dynamic perspective, realism versus excitement (cheating proportions) and other sketches from the project that this sketch will play off of or create new design directions.
These early sketches need to be free and spontaneous. Too much thought, care, and precision at this point only stifle new ideas and restrict free thinking. Good things can happen by accident if you let things go. As a habit, I tend to draw with too much precision, too much care, so I find it helps to loosen up by drawing very small which increases the looseness proportion versus the size of the sketch (if that makes any sense). Since I have a computer and printer at my disposal to resize and manipulate a series of sketches into the same format later (usually 17" wide), I do not concern myself over the placement on the page or notations or studies around the sketch. This is an extension of the proverbial "napkin sketch" only with better paper and art supplies, but the idea of spontaneity is the same. You could easily crank out a series of sketches on the plane or sitting in a chair with a pad of paper in your lap.
The linework above took less than two minutes and is intentionally rough just to lay in general perspective and proportion. Make sure the linework is light on the paper so that you can erase most of it later. BE SPONTANEOUS! Listen to good music, that always helps me. If you are getting stuck for ideas, it helps to alter an element of the process - maybe sketch with a different tool, try a completely different perspective, different paper, or even go to a different location and sketch. If you do the same thing repeatedly, your mind sometimes learns the process too well and things become automatic - new designs won't come to you unless you throw your brain something new to deal with. This is just my theory, but it seems to work for me. If you are drawing the same car over and over again, quit and move to a different company!
In the next step I have moved to a razorpoint fineline black ink pen and put down more deliberate lines to clarify the design. I use the pencil lines underneath as a rough guide and look to see what is working and what is not - for example, I decided to fix the right rear corner perspective a bit. This step might take longer than the first step of sketching in lines with pencil because you have to take a little more care to get the lines right as there is no erasing from this point forward.
The next step is to begin adding highlight areas with white Prismacolor pencil (if you are drawing on white paper, you can't do this.) But first, I make sure to erase the original pencil lines as they will only get in the way and contaminate the colors that go over them- notice how a little bit of pencil work remaining on the rear decklid has given the white prisma in this area a bluish tint. Where you put highlights is very simple- areas facing up are lighter than those facing down. Don't use too much white pencil - there is often a tendency to try to 'finish' the sketch with every step; it doesn't look done NOT because it needs more white pencil, but because there are steps remaining - if you put down more white pencil, there won't be any room for marker and other colors.
Now I have begun marker work with a medium value (5) warm grey alcohol base marker. This one is on its last legs so it provides for some nice gradations. It goes pretty quick. Keep the same things in mind as with the white prisma- be spare with the medium and don't try to finish the drawing here!