Letter writing manuals were popular throughout Jane Austen’s lifetime, and it’s possible then that Jane Austen might have had access to one. Letter writing manuals contained “familiar letters on the most common occasions in life”, and showed examples of what a letter might look like to people who needed to learn the art of letter writing.
From: https://blog.oup.com/2014/03/jane-austen-letter-writing/
You might like a similar (contemporary) book. Gift of a Letter by Alexandra Stoddard. I’ve referred to it on occasion for inspiration.