The story pyramid of: who, what, where, why and how applies to letter writing, especially if you report something, for example a complaint.
Tell your side of the story in some detail, chapter and verse, as much as you can without repeating yourself or whining. Reporters report, they do not have an opinion. Let the reader come to their own conclusion, but with all the help you can give them to favor yourself.
But equally important are the Four Cs: clear, concise, courteous, and complete. When you have a complaint, it’s no use to anyone if you just rant and rant. There must be a series of events that have led to the current unhappy situation. So, start at the beginning and proceed step by step to the end.
Be Sure, don’t leave anything unexplained or overlooked. If you do, that omission could sink you later. So lying is not a good idea, nor is it evasive vagueness; explain everything clearly.
Spelling it all out means it’s complete – answer everybody the questions in your letter, don’t leave them for someone else to pick up and use against you.
Don’t throw angry accusations, everyone hates complainers, watch Donald Trump and Alan Sugar on their respective shows. The newbie. Be courteous to the wrongdoer and your reader, who represents the wrongdoer with far more loyalty than his unwanted intrusion.
By avoiding rudeness and resentment, you build sympathy, at least enough to get a fair hearing. You come as the good boy who was wrong, but he’s reasonable about it.
Finally, be concise. Once you’ve said what you need to say, stop saying it. Repetition sounds like complaining, don’t do it. Once your letter is ready, leave it for a while before editing it again. And keep a cool head when you do. The shorter a letter is, the more likely it will work for you.