I teach letters to the editor the same as any academic writing:
select a relevant topic; gather effective support for your position; state a purpose for writing in a clear, well-placed thesis statement; write an intro that states the issue and your position; be sure your audience can relate to the issue; develop reasons and use rhetorical devices to support your position; write a conclusion that reinforces your thesis; rewrite to correct content and mechanics. It's usually addressed: "To the Editor:"
The best thing to do is to find a letter that has been written and imitate it. Good luck.