Highlight your deep-sea focal point
1 I start off with a rough sketch and some colours, to see what direction I want to take the illustration in. At this point I have four layers: the background, the sketch, the flat colours for the mermaid and the light. Separate layers make it easier to make adjustments. Later on you can flatten the layers as you see fit.
2 Keep the mood of your picture in mind. To make the most of your light source, keep everything around it rather dark. Tweak this effect by making use of Layer Adjustments (Image>Adjustments) to darken or desaturate different parts of the image or by using Layer modes, such as Multiply, and a Soft brush.
3 I kept the direct light on a separate layer until now. Once I’m happy with the colours in the scene, I flatten everything down and start detailing the face and the parts that are illuminated by the sphere. I also put more work into the light source itself and add the rays of light on a Soft Light layer on top.