Step-by-step: Bring in directional lighting
I start by painting the environment using local colours only, without the effect of directional lighting. I sometimes start with darker or lighter local colours to set the initial tone of the image, such as a dark, mysterious scene or a light, hopeful one. Only then do I add the light source…
I start with a Levels adjustment layer to brighten everything up, then invert the layer mask and paint into it to create the light source and areas of bounce light. I then use an Overlay layer to paint the triangular beam of light and add some soft cast shadows around the room.
This strengthens the direction of the light. To add atmosphere to the light, I use a Color Dodge layer to paint in some specks of dust lit by the stream of light. I also use this layer to add hot spots of lighting to other areas of the room that are being directly hit by the light.