Start with the first question
Imagine a new team member opening the project while a customer waits for help. They need to know what the system does, where it runs, and who owns the important decisions. Begin the handover there. A short overview with working links is often more useful than a folder full of notes without an obvious starting point.
Record the ordinary work
Explain how to run the project, release a change, check a failed job, and restore normal service. Write down prerequisites and the signs of a successful result. Keep passwords and access tokens out of the document; describe the approved way to obtain access instead. Include who can authorise changes when an unfamiliar situation appears.
Explain why a decision was made
A future maintainer may wonder why a simple-looking feature has an unusual rule. Record the reason, the alternatives considered, and any limitation that remains. Separate current instructions from historical discussion. Otherwise, someone can follow an old workaround without realising the underlying problem has already changed.
Test the document by using it
Ask a colleague who did not write the system to complete a small task using only the handover. Watch for missing steps and unclear names. Update the notes as part of the same work that changes the product. Documentation is ready when another person can use it, not when the document has an impressive number of pages.