[header] How to write a dear john letter [title] Decide whether you want to a) hurt the other person a lot, or b) cause them minimal pain. [step] That's it. You only get those two choices. 
[title] If you choose a), the letter is easy. [step] Just list their bad points in as clever and witty a way as possible and make it clear you are rejecting them as a potential lover because you can do better.