Introduce the background
I want a simple, out-of-focus background because the only focus of the picture is the ship itself. I use colours already present in the ship, and draw bold shapes and alternate darker and lighter values before blurring the background using Filter>Blur> Gaussian Blur. I add gradients on top to describe the ground plane and add to the sense of atmosphere.
Choose the light direction
To keep things simple, I decide that sunlight comes from above the image. Following what I think my forms are, I paint light on the planes facing up and shadow on the planes facing down. If you’re unsure about lighting you can render a simple sphere in a matching lighting scheme and then reference it when painting everything else.