Well, that usually depends on how much the client pays and how vague their idea is. If payment is the minimum and the idea is general, i do a photobash and paint on top adding ideas i think would work and hoping the perspective works.
If it´s more expensive and clear enough to compensate for the time, i draw and paint it all from scratch.

To decide what i will paint, i choose the pallet before i start painting and make sure the armor reflections mix with the type of pallet i picked.
The best way to decide a background when you don´t know where to start is looking for inspiration at people who have solved similar issues or find a proper photo and imagine what it needs to be added to mix with your imagination.