Here is how a do my 3D backdrop.
I use 3mm (1/8”) Masonite (hardboard), makes no difference which side I use for my backdrop.
I cut it to the shape of the mountains that I want as well as the length.
Start with a coat of earth colour latex (PVA) paint over the whole area.
Mark out with a pencil where you want the closer hills and the hills further away.
Paint on white glue in a thick consistency starting at the bottom.
I use 4 colours of the ground foam from darkest to lightest.
Start at the bottom (closest) with the darkest green and work your way up using a lighter colour as you go.
Remember that the closest mountains are always darker that the furthest ones.
When all is dry, I mark out the different hills just to enhance the look of distance between the hills.
The photos show how it was done on 3mm thick Masonite for a section of my backdrop behind and under a bridge section.
A big thanks to Brian! (His original post is here).