The better writing lesson I ever had was in the Army. Army correspondence is simple. Here’s a description of the standard arrangement:

Para. #1 — This letter is about [subject].
Para. #2 — This is the situation or problem.
Para. #3 — This is how we need to fix it. (This includes either your plan of action or a request for assistance.)
Para. #4 (if necessary) — These are the consequences of not addressing this issue.
Para. #5 — If you have questions, contact [name and phone].