A good farewell letter is not the one that says the most. It is not the longest, the most polished, or the most theologically thorough. A good farewell letter is the one that sounds like you, about them, in a way that no one else could have written.
That is it. Specificity and voice. Everything else is optional.
Be specific
The single most important thing you can do in a farewell letter is name the details. Not "I loved our time together" but the actual time, the actual place, the actual thing that happened. Not "you were always there for me" but the specific Tuesday when they showed up, what they said, what it meant.
Generalities comfort no one. They sound like greeting cards. The person reading your letter does not need to be told that you loved them in the abstract. They need to know that you remember the afternoon they burned the rice and you both laughed until you could not breathe, and that you carried that afternoon with you every day after.
The details are the proof. They are the evidence that the love was real and particular, not a sentiment you could have directed at anyone.
Use your real voice
The second mistake people make is formality. They sit down to write a farewell letter and suddenly they sound like a Victorian solicitor. "Dearest beloved, it is with great tenderness that I pen these words." No one talks like that. You certainly do not talk like that.
Write the way you speak. If you swear sometimes, it is all right to let one through. If you are funny, be funny. If you are awkward, be awkward. The person reading this letter wants to hear you, not a performance of seriousness that you would never deliver in person.
Read your letter aloud. If it does not sound like you — if you would not say these sentences across a kitchen table — rewrite it until it does. Paul wrote his farewell to Timothy in exactly this way — personal, warm, unmistakably his. What Paul wrote knowing he might die is one of the finest models of voice in a farewell letter.
Say the thing only you can say
Every farewell letter has generic content: I love you, I will miss you, be strong. These are fine. They are not what the person will remember.
What they will remember is the thing only you could have said. The observation only you made. The memory only the two of you share. The quality in them that you noticed and no one else mentioned. The inside joke. The nickname. The moment.
Ask yourself: if someone else wrote this letter, would it still work? If the answer is yes, you have not gone deep enough yet. Keep writing until the letter could only have come from you, about this person, in this voice.
Keep it shorter than you think
The impulse is to say everything. Resist it. A farewell letter is not a biography or a comprehensive account of the relationship. It is the concentrate. The single truest thing, said as clearly as you can say it.
One page is more than enough. Half a page can be perfect. The person reading it will read it dozens of times, and each time they will find something new in it — but only if it is focused enough to reward that kind of attention. If you need a step-by-step start, how to write a goodbye letter walks through the basics.
The only test
When you finish, ask one question: will this person feel known? Not admired, not praised, not lectured. Known. If the answer is yes, the letter is done. And do not be surprised if you cry when you finish writing — that is how most people know they got it right.
Write a letter that sounds like you — because your voice is the part they will miss most.