Think about a topic for your door, mine will be a creepy Halloween door, for example. You can find inspiration here.
Start painting the parts first.
So that the door can be opened, you should play the "hinges" round. It works best with a nail file. On the back you stick the small bridges, put on the door and stick the door frame on the front. Pay attention to it that where the "hinges" should rotate should not be a glue if possible.
Glue the stairs together and design them according to your wishes.
Before you glue the inner box together, think about where the lighting should be so that you drill suitable openings beforehand.
Now you can connect stairs and door to the box.
Lay the cables of your lighting before you stick the outer walls. Finally, only stick and design the front.