Okay, who writes the sponsor letter? or who do you recommend to write it?
Your sponsor writes the sponsor letter... as it is their confirmation of support for the application.
It should include:
Paragraph 1: how they are eligible to be a sponsor (their UK citizenship)
Paragraph 2: their employment details and how they are meeting the financial requirement, including which category they meet
Paragraph 3: where you will live together in the UK, including property address, how many bedrooms, whether it is rented, owned by them, or owned by family/friends who are letting you stay there
Paragraph 4: a short, factual history of your relationship (no personal feelings) - so, where and when you met, where and when you entered into a relationship, where and when you got engaged, where and when you got married, how often you see each other, how you communicate while apart, and why you are choosing to settle in the UK and not the US