Thought I could do one of these again, if anyone's interested in the process.
The software is Paint Tool SAI 2.

1. Rough sketch. Drawing up perspective and the basic shapes.
2. Detailed sketch. Defining everything.
3. Inking. The colorization of the lines was made after all painting, though.
4. Blocking in background colours on its own layer.
5. Characters colour/flats. Usually I paint directly on this layer, one colour area at a time, but for this piece I went about it a bit differently, as seen in the following steps. I've just added some colour variation to the skin here, to make it a bit more alive.
6-7. Shade layer. I started by blocking in the square door shape, then adjusted it after the shape of the bodies and objects. This layer is set to "Multiply", which makes it act like a shadow that blends together with the layers below. I then added the shadow colour to all other areas that would cast a shadow from the light source direction. I also added a tone to make the shadow a bit lighter closer to the opening and darker further away, and some coloured bounce light here and there. Most of the painting work was made in this step, and layer!
8-9. I made a new layer for some secondary light and definitions. This was set to "Soft Light".

10. My usual pee process. Trails and wet cloth areas are painted on a "Multiply" layer.
11. Then flowing fluids and highlights on a normal layer.
12. Because I had the multiply shade layer this time, I needed to make some additional highlights on a separate layer above it for the parts that are partly in light or passes a shaded area. The same was done for a few details on the cum version.

And done! This method can actually be a bit faster than painting everything normally, especially if there's a strong light effect like here. I wasn't sure how it would turn out from the beginning though, and just kept experimenting with different colours of shade and light until it looked right to me. I hope you enjoy this piece!