Sometimes the best way to tell someone how you feel about him is with a scathing letter. When you write a scathing letter, you can take the time to get your emotions under control and think of exactly how you feel about someone. More importantly, you can also think of, and write, why you feel that way. This is much easier than telling someone this information to his face because it's so easy to lose your temper and, with it, your logic and rationality.
Jot down all the facts of the event or relationship about which you want to write. It is important that you don't leave anything out when you write your letter, so take a piece of scrap paper and write every single thing down. There's no need for correct word order or anything like that; right now, you're just making sure you have everything in front of you.
Note how each of those facts affected you or made you feel. This will clearly show the link between actions and consequences so that the person you are writing to knows precisely what she has done.
Draft your first paragraph of the letter. The first paragraph should simply introduce the letter and say why you are writing it and to what it relates. So, you could write "Dear Jane, I'm writing to tell you how you made me feel when you..." Don't include any facts at this stage; the first paragraph is just an introduction.
Write a paragraph for each of the facts above. You should open each paragraph with the fact, then follow it up with the feeling. This will show the person exactly what he has done and what the effects of that were.
Conclude your letter with a paragraph that sums up the main points. You should include a broader theme about what the overall effect of all of the person's actions was.
Re-read the letter to make sure it is clear. The sentences and paragraphs should be short and sharp in order to drive home the letter's key points.