I will have to say that it has to either start from research or from a
simple idea. You need an idea to start, you can get it while researching or from your imagination.
OK, The idea itself is what makes a good diorama and a bad diorama. The idea has to be able to be represented on a diorama as there are many ideas that are just out of proportion.
Also, your idea must be strong enough, it should give the viewer a message, if it doesn't then it is just a model display.
Once you have your idea it is time for more research. Look for what you will need for your diorama, and get it. Make vehicles, houses, and figures similar to a common time or theatre of war.
A common mistake is to try to make a diorama with something that you
already have. This is again a display and not a diorama. Of course there is nothing wrong with using some of the stuff you have if you have a big stash, but don't try to use a king tiger for an early 1941 diorama just because that is what you have. (don't put Germans in guadalcanal!!!)
Good luck