first of all, you need a good model: a good angle, nice details, and a good contrast.
for example: lotus 340r
http://www.340r.net/pictures/ollie/nearside.jpg
once its done, try to draw the principal lines (take your time cause its the hardest part) the proportions must be prefect.
if you want to make a costum bodykit os something like that, now is the time.
Once its done, draw the headlamp. dont press too hard on your pencil.
when you are glad whit it, add contrast (now its time to press on the pencil) until its the same as the pic.
Start drawing the body and do the same for the rest of the car
use a klenex to remove all imperfections (but dont use it where there is a lot of detail) now that's as soft as baby skin, take an eracer and erace where the light is. Darker where the shadows are.
thats it.
i will do a viper and scan it on every steps.
http://www.supercars.net/Pic?s=1&id=...TS-R%20Concept