My top down approach for writing software design docs: 1. Business objectives 2. Users and use cases 3. Architecture <- design decision, performance expectation

My top down approach for writing software design docs: 1. Business objectives 2. Users and use cases 3. Architecture <- design decision, performance expectation, wide picture of the system 4. Infrastructures <- you may mention environment and networks here, like k8, aws or gcp. 5. Application <- ui, flow, activity diagram, algorithm here 6. Data <- now you can place UML here As for code level documentation, writing readable codes is enough as documentation. If some guys can understand what the codes do by reading the codes, documentation won't be needed. How to write readable codes? A good reference for this is Clean Code book. If documentation is really needed, Java Doc style is worth to follow. Hope this helps