Early in my career, I started off as a graphic designer. So creating box art and displays was something I did on the daily. When it came time to make some diorama accessories, my prior experience translated well. For the most part, the artwork I would create was the same. It was the scale that would change.
Also in the past, I did a few semesters in Classical Animation. And when creating backgrounds for animation, the biggest takeaway was adding details. It is the same with dioramas. Fill it with all sorts of details. You want it to look and feel like the space is being lived in.
What kind of details?
Anything and everything. Imagine a bedroom, now imagine the person that lives in that bedroom. What kind of things do you imagine seeing in the bedroom? Everything you add is an extension of the person or character that lives in that space. So, if the character is a teenage boy, you would see a number of objects in the room that scream teenage boy. A skateboard, clothes on the floor, iPhone, sketchbooks, etc…I think you get the idea. The more you add to the room the better. You will begin to notice that every object in the room starts to tell a story. A good diorama does exactly this. It tells a story.