Step-by-step: Blocking in your light sources
The sky is my main light source. It’ll affect the whole lighting scheme, the overall colour scheme and mood of the scene that I’m painting. I want to use only one light source and so I imagine a scene after sunset, which leaves the foreground without almost any natural light.
I darken the environment using a Levels adjustment layer and add a midtone orange colour on an Overlay layer. Then I introduce my fire on a Screen layer to the scene and erase back the layer mask on the Adjustment layer at the surfaces, where the light affects the environment.
I create an extra one-point perspective grid, where the mid-point meets my light source (the fire). This helps to see how the fire affects its surrounding and what direction the firelight takes. This enables me to paint accurate cast shadows on the ground, caused by the added rubble.