Answer:
A letter for any purpose, including an excuse letter, should include the following:
Date of your letter.
Name of person the letter is to.
Address of person the letter is to (unless you will hand deliver the letter).
The salutation, "Dear ..."
State the reason you are writing.
State any details regarding the reason you are writing.
If applicable, tell what you expect or what you want the recipient to do.
The closing, "Sincerely, or With Regards, etc."
Your signature.
Your name printed or typed.
If applicable, your title.