Your bees need space to store honey. Normally in the wild, the bees would simply build more comb. However, in a managed colony, you need to provide the space as the bees require it. It is unlikely that your bees will need a super when you first inspect them but in the spring you should always have supers ready in case they are required.
Preparing The Supers
You can assemble the supers with the frames inside ready to pop on. You should remember that the supers will be holding honey that you will eat - so you should try to keep them 'food safe' (i.e. not put them directly on the ground and get them covered in mud).
Top Or Bottom?
You can either put you supers above or below other supers.
A queen excluder stops the queen and drones from walking up into the supers. This stops her laying in the supers which are designed for honey storage. You might not need a queen excluder when using the Beehaus because the brood area is sufficiently deep to hold all the brood. However, just in case, some are provided. When you first put a queen excluder on, the bees might not initially move through it. (Note: in winter you must make sure that the queen excluder is removed - to ensure that the bees and queen can get access to the honey stores without hindrance).
Can You Add Supers Too Quickly?
Yes. If you provide the bees with top much space, you increase the amount of air that they have to keep warm and you also make it harder to inspect the hive by having to handle a lot of unused boxes.