theres a way to use layers to make it look better and easier but i dunno how to use it yet...
1) i suggest finding a picture that dosnt have to many shadows or lines because then u have more to deal with...
2) pick the "Airbrush tool (J)" and set it to the thickness u want it to be and make sure the color is black.
3) zoom into the picture and start to trace over the lines and cracks of the car. (ex: the small crack between the fender and hood)
4) after ur done with that, pick the "Lasso tool (L)" and create a set area to color in
5) then use the "Eyedropper tool (I) and pick a color within the area u set with the lasso tool
6) then use the "Gradient tool (G)" to set the color in the area thus making it turn into a smooth even color
thats how i did but
i found another way to do it online but im not really familiar on how to do it that way...