I did this by marking out my robot with thick black lines, i then filled in all the main body parts with a light grey and then i shaded the right sections of the outline in a darker grey creating the illusion of shade and also form.
This shading method is used in multiple forms of media.
it is used in games such as:
Dragon ball Xenoverse
Naruto games
Borderlands 1 & 2
and in films and TV shows such as:
The Simpsons
Family guy
Archer