The best way to write ANY letter, formal or informal, is to do the following:
1. Consider what the purpose is. What do you want the person to DO after they have read your letter. Donate to your worthy cause? Send you a signed photo? Understand your point of view? Laugh? Be informed?
2. Consider your audience. What does this person know already? What background does this person have? Are they likely to look favorably on your request, if there is one? Do they even know who you are?
3. Consider what the main messages are. You can only answer those after you've figured out the first two. Write down the three main points you want to make.
THEN you can start your letter (or report or essay or whatever you're writing). Don't get so caught up in "Leave two lines, indented five spaces" type issues of your letter that you forget to think about why you're doing it in the first place.