Each paragraph should contain one idea and serves as a mini story.
Look at the example and you’ll see when it is natural to start a new paragraph.
• The first paragraph can be used for thanking someone, asking how someone is doing, telling them how you are doing, etc.
• The last paragraph is used to make arrangements, send regards to others
• The other paragraphs deal with the real subject matter of the letter; usually the reason why you are writing. This can be anything: a problem, a request, information, etc.