Even for experts in personal knowledge management it is hard to explain in a single sentence how to create a Luhmann style note card system. One of the best papers about the subject was written by Schmidt [1] but it was written for an academic audience. In contrast the picture shown on top of this blogpost summarizes the idea in a very compact format.
A luhmann note taking system is basically a mindmap. In the mindmap a hierarchical structure is available what is common for all mindmaps. A node can have subnodes which are linked with arrows. Mind maps are a well known creativity technique but the system can't scale up to larger needs. Writing down 20 nodes on a single sheet of paper is maybe possible but writing down 100 and more contradicts the idea of a mindmap.
The Luhmann notetaking system takes a mind map as starting point and projects the nodes on a 1d array which is the notebox. the 1d array is a storage container for a hierarchical mindmap. This mapping allows to create larger mind maps which have unlimited amount of entries. in theory it is possible to onvert a Luhmann note box back into a mindmap and print it out but the graph would become too large. What a user is prefering instead is to work only with the 1d array. Instead of creating new entries in the original mindmap the topics are created in the card box.
Larger mindmaps
... can be mapped also to a 1d array. The figure looks a bit messy but from a technical perspective all the arraws to mapped to a single target location. The idea is that the same semantic network is rendered either as a mindmap or as a 1d array. The 1d array allows to store the information more compact and it is easy to use paper based index cards for physical storage.
The unfolded card box is a normal mindmap. And if the amount of cards is higher it will become a very huge mindmap It would be equal to draw a mindmap on an entire wall. And if the wall #1 is running out of space the next wall is added. The interesting situation is that the entire system remains always a mindmap. So the shoebox of Luhmann which was in the reality a huge cabinet is holding a mindmap. And the mindmap is the second brain.
The example mindmap in the figure consists of only 15 nodes. After adding additional 15 nodes, a normal A4 sheet of paper will run out of space. This might explain, why only smaller mindmaps are drawn in the reality which makes it not practical for complex topics. It is not possible to write down 100 kb of notes into a mind.
At least this is the hypothesis if the luhmann numbering system is unknown. After mapping the mindmap to a 1d array it is pretty easy to add additional nodes and there is no upper limit. So the Luhmann method is a super-mindmap.
References
[1] Schmidt, Johannes FK. "Niklas Luhmann’s Card Index: Thinking Tool, Communication Partner, Publication Machine." Forgetting Machines: Knowledge Management Evolution in Early Modern Europe. Brill, 2016. 287-311.