How to write one:

VIDEO from the ABC on how to write a formal letter, including for job applications or to argue a point of view. https://www.abc.net.au/education/learn-english/everyday-english-writing-a-formal-letter/9815732

More basics of how to write a formal letter: https://www.bbc.co.uk/bitesize/topics/zv7fqp3/articles/zkq8hbk

Templates and Examples:

A very basic example opinion letter is here: https://k12.thoughtfullearning.com/studentmodels/letter-editor This is Grade 4 or 5 level, so I hope you would do more details than this.

Get a Word or Google Docs template for a formal letter here: https://www.thebalancecareers.com/sample-letter-format-2063479