For me it is either
I will think of a design in my head and think about it, see some shape or pattern somewhere and mentally log it for a skin o r browse the skin section of this forum or master skinnerz garages for inspiration.
Sketch ideas on paper, just shapes and similar before making it in photoshop.
Search google for race cars for inspiration or for a replica.
Or finally, pick a main sponsor and make a design around the sponsor name or logo.