How I begin:

Just to preface, this is NOT a tutorial, but MY step by step process. 

Thumbnailing

I usually start off with a couple quick thumbnail sketches, usually creating three at least before deciding on the final composition. 

I try to usually keep my lines loose during this process. 

Clean-up Lines

From there I clean-up the lines just enough that I can already start placing down colours.

I don't always add lineart to my more illustrative pieces. This is just a personal preference and is mostly for giving myself less work to do.

I only try to have clean lines for the face and hair. I don't try to be overly detailed with my clean up lines anywhere else, unless I really need to.

Flat Colours and Shading

When it comes down to laying out flat colours, I usually find references from photographs of places and environments, since they usually have the most cohesive pallets.

But for this piece, they have been characters I've been working on for years, so I have already created pallets for them. It was mostly the lighting that I needed to find references for.

As for shading, I start with blocking in the lights and shadows with different layer modes, not really thinking about refining them until rendering. 

I just use the multiply for shadow and add glow or colour dodge for highlights. Playing around with the opacity until I am satisfied with where I can start rendering. This is just a place holder for where lighting goes in the piece. 

I just want to get the feel for where the shadows need to be emphasized and what the end product could be.

Rendering

This is probably the most time consuming part of my process, where everything gets refined and when I want to cry.

I start with the face and expand outwards, but still hopping from one area to another when rending because I cannot focus on one spot. 

I always try to turn on and off the lighting layers, because they are just place holders for where the lights and shadows goes.

The scary lighting layer that needs to be blended in

Finalizing the Piece

From "that's doesn't look right to looks good"

I just use more layer modes, play around with the contrast, and check for any inconsistences in the piece. Just trying to put the whole piece together. Blending, refining, cropping and finalizing until I am satisfied with the piece.