A single place to look
When someone dies, the people closest to them are handed a second job on the worst week of their lives: find the accounts, the policies, the passwords, the wishes. Most of that information is not written down anywhere. It lives in one person's head, and that person is gone. An end-of-life planner is simply the one place everyone can look — a physical book that answers the questions your family will otherwise have to guess at.
What belongs in it
Think in three layers. First, the practical records: identification, key contacts, medical details, insurance, bank and utility accounts, and where important documents are kept. Second, your decisions: funeral wishes, arrangements for children or pets, what should happen to your belongings. Third, the personal: letters, gratitude, final words. A good planner gives each of these a page with prompts, so nothing is left to memory or improvisation.
When to start
The honest answer is now, while it feels unnecessary. A planner is easiest to fill in when there is no pressure — an ordinary afternoon, a cup of tea, no deadline. Start with the practical sections you already know by heart; the personal pages can wait until you're ready. What matters is that the book exists and that someone you trust knows where it is.