You've already chosen the setting for your diorama, next you need to know what size/scale you want it to be. You'd need to have a baseboard to set your diorama on.
Here's what I do,
1. Use plaster of Paris or filler to form the contours.
2.Paint the entire section with a Matt earth colour then use PVA glue diluted with water and a drop of washing up liquid to apply scatter materials.
3. Form buildings and structures out of plastic card or balsa wood.
You can also buy weathering powders to add dirt and grime to structures.
To be honest the best place to look for advice on modelling techniques is on YouTube, the best models are to be found in the model railroad vids. The techniques used for dioramas are exactly the same as model railroads.
There are plenty of companies on line and in model shops that stock the scatter materials.
Research is the key, get as many reference photos of your chosen site as you can.
Take a look at this vid and you'll see what you need for a diorama.