Imagine asking another capable person to continue your personal project tomorrow. What would they need to understand its purpose, current state and next useful action?
Write that short handover even if you expect to remain the only person doing the work. It reveals gaps that familiarity can hide: an undefined audience, an undocumented assumption or a decision that has never actually been made.
Include the working route
For a fictional guide, record the intended reader, the example already tested, the sections awaiting review and the location of source material. Identify the next action precisely enough that someone could begin without interviewing you.
Keep alternatives and abandoned versions clearly labelled. Otherwise, the handover becomes a puzzle about which document contains the current agreement.
Use the note when returning after a break. If it fails to answer a practical question, improve it while that confusion is fresh. This creates continuity without requiring an elaborate management system. A project becomes easier to own when its essential reasoning can survive an interruption in the owner's immediate memory.