How do you make an indoor map?

You mean like inside?
You either need to take and place models from the shipped indoor maps(Mustafar, Polis Massa etc..) or model your own.

What do you mean? Is it hard to make one?

Indoor maps are very hard to make with the given assets. Usually they'll look unprofessional and buggy.

The only way to make your own indoor map is to take up modeling and either modify the assets so they will go together nicely or make completely new models like in Naboo: Theed Hangar and Naboo: Sewers.

ok thanks. lock it up!