Step by step (Patreon)

Sketch!

Lines!

Shading/texture on a linear burn layer.

Hid the linear burn layer and did some very rough colouring on a different layer. 

Added more detail to the colouring, mostly using the colour picker to grab colours from other parts of the image and blend.

Unhid the linear burn layer and put it on top of the colouring layer.

I then duplicated the colouring layer, put the duplicate on top of the linear burn layer, set the opacity to about 20% and locked it to the linear burn layer underneath it.

This adds some really nice texture/shading to the colouring, with minimal thought/effort on my part :)

I didn't put any thought into what I wanted for the background which was a terrible idea. Eventually settled on this?

Painting on new layers, on top of the foreground layers, adding detail

A few more details, a levels adjustment layer, and and an overlay layer on top of everything with dark/blue purple airbrushed over the darker parts, and light pinks/oranges airbrushed over the lighter parts, and done! I'd originally planned on keeping the colours muted and sepia-ish but apparently I like bright colours too much.

Please ask if you have any questions :)