Here I started the basic shading. I slipped onto Photoshop to do this. No specific brushes are needed, I simply went onto a new layer and starting adding shadows and highlights where I saw fit. The light source came from the top right corner so all shadows faced opposite. Once I had the scribbles in, I began to use the smudge button as well as blur until everything had the same overall 3D appearance. I must point out that here the detail does not matter much as it will be a night time image and most of the background won't be visible.