You’re going to need to be more specific in what you’re asking. Different types of letters have different structures and, generally speaking, there are no hard and fast rules. There may be requisite content that needs to be covered (e.g. A letter of demand, or a concerns notice for defamation), but in most cases the structuring of that is a personal question.
There are many ’functional’ examples of letter precedents available if your search for them, or in precedent packages. Many of these I would consider to be passable, but poor in terms of their structuring, grammar and language. I’m sure there would be practitioners who would consider the same of mine.
Learning to write an effective letter is an art form, and it takes time and practice.
So, the question is, what are you specifically asking for?