Purpose of a Friendly Letter
A friendly letter (or informal letter) is a way of communicating between two people (sometimes more) who are usually well acquainted. There are many uses and reasons for writing a friendly letter but friendly letters will usually consist of topics on a personal level. Friendly letters can either be printed or hand-written.
Friendly Letter Writing
The friendly letter is typically less formal than that of a business letter. Usually the first paragraph of the body will consist of an introduction which will give the recipient an idea about why you're writing to them with a short summary of the main topic of your letter. If you don't know the person you are writing to, you may want to introduce yourself in this introductory paragraph as well.
The next few paragraphs will usually consist of the message you want to get across along with any details you may want to convey.
The last paragraph will usually be the conclusion where you wrap everything up. You can sum up your main idea in this paragraph, thank the recipient for their time, wish the recipient well, and/or ask any questions.
Since friendly letters are less formal, you can feel free to write it however you like, but the above format is fairly common.