Start with the moment, not the person
A blank page addressed to somebody you love is paralysing. There is too much to say, no obvious place to begin, and an unspoken sense that whatever you write ought to be worthy of the whole relationship. Most people write two sentences, feel that they have failed, and close the laptop.
A blank page addressed to a specific afternoon is a completely different task. Open on the morning of your wedding. Open when you move into your first place. Open when you are having a year that will not let up. Open when you are about to become a parent.
The moment does the hard work for you. It tells you what tone to use, what they will already be feeling when they read it, and what they might need from you that particular day. A wedding morning wants warmth and a bit of humour. A hard year wants steadiness and permission to struggle. You are not trying to summarise a relationship. You are writing to one afternoon, and one afternoon is a problem small enough to solve.
Choose fewer moments than you think
The temptation, once the idea takes hold, is to cover everything: every birthday, every milestone, every possible turn a life could take. It is worth resisting, for two quite different reasons.
The first is simply quality. Twenty thin letters land with far less weight than four that were clearly written with care. Volume reads as obligation. A small number of letters that are obviously considered reads as love. If you find yourself writing the same encouragement for the fourth time in slightly different words, you have passed the point where more is helping.
The second reason matters more. A message written for a moment that never arrives, or arrives very differently, can be painful rather than comforting. A letter for a wedding that does not happen sits unopened as a small permanent reproach. A letter for a graduation they never wanted, or a child they could not have, is an unintended sadness delivered by somebody who is no longer there to soften it.
So pick moments that are likely rather than aspirational, or write them broadly enough to survive a life that took another shape. "Open when you find your person" survives more futures than "open on your wedding day". "Open when you make somewhere your own" survives more than "open when you buy a house". A small change in framing buys a great deal of resilience.
“Write to one afternoon in their life, not to the whole of it.”
Be specific, and let it be ordinary
The parts of these letters that people reread are almost never the grand statements. They are the small, exact details.
The way you always got the pronunciation of a particular word wrong, and refused to be corrected. What the kitchen sounded like on a Sunday morning. The argument about a film that neither of you would let go for eleven years. The specific noise the front door made. The fact that you always cried at the same advert.
Specific things prove you were there. Advice ages badly, because the world it assumes stops existing. Sweeping declarations flatten out by the third read, because the reader already knew you loved them and the sentence adds nothing new. But a detail only you would have remembered does not fade, and cannot be obtained from anywhere else. It is the closest thing to your actual presence that survives on paper.
If you are stuck, stop trying to write about feelings and describe a room instead. Describe what they were like at nine years old. Describe something you noticed about them and never mentioned. Describe a day that was not important. The feeling arrives on its own, carried in by the detail, and it arrives far more convincingly than if you had aimed at it directly.
Do not make it a final word
The heaviest trap is trying to write something conclusive. A letter that is attempting to be the last thing you ever say tends to come out formal, careful, and slightly false. You can hear the effort in it. Worse, it puts a strange weight on the person reading, who now has to receive your Final Statement rather than simply hear from you.
Write it like a note left on a table instead. Warm, unfinished, a little casual, allowed to end mid-thought. You can always write another one. Nothing about this has to be complete.
The aim is not to close anything off or to sum anything up. It is to be in the room for a moment on a day when they would have wanted you there. A note that sounds like you actually talking is worth more than a polished paragraph that sounds like a eulogy you wrote for yourself.
It also helps to say plainly that you did not know how their life turned out. Acknowledging the gap makes the letter honest rather than eerie. "I have no idea what you are like now, but I would guess you still hate being late" is a better sentence than any confident prediction, and it invites them to smile rather than to feel spoken for.
Write for who they will be, not who they are
A letter for a distant moment will be read by somebody who has changed. The nine-year-old you are picturing will be thirty when they open it. The friend you are writing to has had a decade of experiences you know nothing about.
That means writing to somebody slightly unfamiliar, and it is worth doing consciously. Avoid too much reliance on the current version of them: their present job, their current partner, the phase they are going through. Those references date fastest and can land awkwardly.
What travels well is character rather than circumstance. The things about somebody that are true at nine and still true at forty. Their stubbornness. Their particular kind of kindness. The way they take on other people's problems. Write to that, and the letter still fits whoever opens it.
It is also worth writing as though they are doing fine. Most people, most of the time, are. A letter that assumes they are struggling can feel like a small insult when read on an ordinary good day, whereas a letter that assumes they are well is easy to read warmly even during a hard one.
Practical things that make it easier
Write badly first. The single biggest obstacle is trying to produce a finished letter in one attempt. Get an ugly version down with no concern for how it sounds, then fix it. Almost every good letter of this kind is a second draft of something clumsy.
Keep them short. Three or four paragraphs is usually plenty. Length is not a measure of love, and a long letter is harder to reread, which is the thing you actually want them to do.
Date it, or at least say roughly where you were in your life when you wrote it. A letter that says "written the winter we lived in the flat with the broken heating" gives the reader a place to put you, and that context becomes more valuable the more time passes.
Do not seal them off in your head as untouchable once written. Reread them every few years. Some will still be right. Some will need updating because the relationship moved. Rewriting a letter is not undoing it; it is keeping it true.
And write them earlier than feels necessary. The reason to do it now is not morbid, it is practical: people who wait for the right moment tend to be interrupted before they find it. Almost nobody regrets having written one too soon.