When it comes to assembling a gingerbread house it is so much easier if your baked gingerbread pieces are as straight and flat as possible. Not always easy to accomplish, but it might help with this pro-tip:
When the baked dough comes out of the oven, straight away put a piece of baking paper/ parchment paper over and another baking tray, or something equally flat and heavy - and let cool down.
This ensures that the pieces cool down flat and straight.
I might not seem like a big thing, but it will really make a difference when putting all the pieces together. Especially if you use a gingerbread dough that rises a lot in the oven.
If you are looking for ways to improve your gingerbread house building, this might be the tip for you.
Good luck