Do a Google search for car rendering. Sketch as much as you can, especially existing designs.
99% of getting this portfolio together is your own internal drive. With or without a mentor, you will learn by making 10,000 mistakes and horrible drawings. With your background, you will recognize them and be able to avoid them again.
Ray Jepson914 Blog
"Be orderly in your life, and ordinary like a bourgeois, in order to be violent and original in your works." Gustave Flaubert