So, finally, a friendly professional person installed the door to the shop for me. I have observed this work many times, but I don’t dare to do it on my own. It requires absolute accuracy, precision and, last but not least, knowledge about what you’re doing.

First, one half of the doorframe is glued and screwed together and inserted into the door-hole. After levelling everything carefully with a spirit level, it is fixed in the correct position with several wedges and the hollows are filled with construction foam.

To ensure the correct position of every part of the frame, the whole construction is fixed with tension rods until the foam has cured and hardened. Next, the other half of the Frame will be installed (foam, glue and screws) and the door is ready.

More information and instructions (german language):
https://www.obi.de/magazin/wohnen/innentueren-einbauen
https://www.bauhaus.info/ratgeber/bauen-sanieren/zimmertuer-einbauen
https://www.selbst.de/innentuer-einbauen-9357.html