From chapter 9 of the Robert Collier Letter book here are the six essential elements to a good letter.
1. The opening which gets the readers attention.
2. The description which pictures your proposition to the reader.
3. The motive which creates a longing in the reader’s mind for what you are selling.
4. The proof which offers to the reader proof of the truth of your statements.
5. The snapper which gets immediate action.
6. The close which tells the reader just what to do and how to do it.