Hi and welcome to SMCars.
Since this is a concept, I assume there are no blueprints. You'd want to specify your shape in the orthographic views with splines and curves. Once you have that setup ready, adjust the vertices of your model according to the curves for a quick fix. Define the shape first, and then start creating edge loops and creases.
Also, use the 'Chamfer' option for edges with discretion. While it is useful in certain situations, you might want to stay away from using it too much. Try and use edge loops where necessary instead. This method involves selecting a ring of edges and then using the 'connect' option, wherein you also have the option of sliding it near or away from the shape defining edge as you prefer.
Try and space out your edge loops around the body as evenly as possible to avoid the obvious 'hills and valleys' contour on the body surface.
I am attaching a simple mesh of an old Volkswagen Beetle WIP image for a client from some time back as a mesh-flow reference for you. I hope you find it useful for adapting it to your model.
Cheers,
Sonnu.