I recently had the opportunity to write a breakdown article for my recent project "Ruined Temple" for the awesome people over at https://gamesartist.co.uk
You can find the full post here:
https://gamesartist.co.uk/ruined-temple-recreating-an-18th-century-painting-in-ue4-thomas-bauer/
I talked about every aspect of the creation process as best as I could. If you still have any questions don't hesitate to contact me :)
https://www.artstation.com/gamesartist