Here is a 6-step process of the Marie painting from concept to completion. Forgive the blurriness on the 3rd step - I had to grab it from the stream because I forgot to save a layer copy of the original blue fire/magic before I refined it.
Also included here is the source file for the painting of Marie. You can download the PSD source files here. You can toggle the many blocked out areas as well as her edits. Some layer effects may be different outside of Photoshop due to using LAB color mode.
Don't have Photoshop or software that will open Photoshop files? You can open it with online PSD editors like Photopea. It will also load in GIMP, but it does not support all of Photoshop's layer features.
Source file archive is available here.