At first, treat arc42 section 8 as a checklist and work iteratively:
- Remove every topic that is not relevant for your system.
- Prioritize the remaining subsections, criteria should be importance and risk.
- Work on the highest priorities and briefly (!) document the corresponding decisions.
Many crosscutting concepts will be highly technical, therefore you document or specify those for developers. Source code with brief explanations can sometimes be sufficient - and can save you from writing awkward documents!
(to be done: include diagram from training slides)