Decide who the letter is for
Write to one person at a time. A letter for your partner reads differently to a letter for a child, a sibling, a close friend, or a colleague. Pick one Recipient, and write as if you were sitting opposite them. If you want to leave letters for several people, write each one separately rather than blending them into a single message.
A simple structure that works
Open with one short sentence about why you wanted them to have a letter from you. Share one or two specific memories that show what they meant to you. Tell them what you were most proud of about them, in plain words. Say what you hope for them next, without prescribing how they live. Add anything practical you want them to know: an apology, a forgiveness, an instruction, an unfinished story. Close with warmth, in the kind of words you would normally end a conversation.
How to keep the tone right
Read it aloud once. If it sounds like you, keep it. If it sounds like a speech, soften it. Avoid lecturing; one specific memory carries more weight than ten pieces of advice. It is fine for the letter to be funny, or quiet, or imperfect. People do not keep these letters for the prose; they keep them for the voice.
Length, format, and updates
Three hundred to eight hundred words is plenty for most people. A short voice note or video can sit alongside the letter if you want. Update the letter every year or two, especially after big life events. Date the version so the Recipient knows when it was written.
Make sure it actually arrives
A letter helps nobody if it is lost in a drawer. Three options work in the UK: lodge a sealed envelope with your solicitor and reference it in a letter of wishes; give a sealed copy to one trusted person with clear instructions; or use a sealed-message service like Y.O.D.O. that holds the letter for one named Recipient and releases it only after a Delegate has reported the passing, identity verification has completed, and a 72-hour dispute window has closed.