How to Write a Dear John Letter
With a bit of tact and planning, you can write a well-worded Dear John letter. In the first sentence, make it clear that the relationship is over, so you don't leave them guessing. Say something like, “I'm sorry, but I don't think we should be together anymore.” Then, give them a simple explanation for why you're breaking up. If you want to keep things amicable, remind them that you enjoyed the time you had together. You can even tell them you still care about them and ask to remain friends. You can send the letter by mail, or by email for an easier option.
Decide whether you want to a) hurt the other person a lot, or b) cause them minimal pain. That's it. You only get those two choices. If you choose a), the letter is easy. 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. You can make it hurt more by suggesting they were really close to being good enough and that they should keep trying and not lose hope. You can specify one of their friends as an example of someone more attractive. Next, you should go on a Buddhist retreat and ask yourself exactly what the meaning of your life is and why you want to be evil. Eliminate all criticism of the person if you want to soften the blow. Save your letter for 24 hours and re-read several times before sending. Keeping looking for hidden criticisms and condemnations. Remember, the person reading it will likely be thinking, "What did I do wrong?" or "What is wrong with me?" Be clear that you want to terminate the relationship. Leave no doubt. Keep the letter short.  Make sure you have privacy and that the person has an escape route if you choose to do it in person. E-mail is probably the best way. Provide one simple explanation about why it is not working and make it about you, not them. For example, "I am not over X." or "I am not seeking a relationship" or "I prefer to casually date many people for the time being." or "I have personal issues that make any relationship impossible." If you cannot bring yourself to say any of these things, then say "I do not feel love in my heart. I don't understand why, but I can say it is not about you." Remind the person that your rejection is not a reflection on them, even though it obviously is.  Mention to the person in what ways they have contributed to you or something positive you learnt from them that you will carry forward the rest of your life. If you can't think of anything, be sure to mention a few good times you spend with them which will provide cherished memories. Wish them luck in future relationships and let them know you still care about them as a person and hope that your friendship can survive.  Do not talk to others about the break up any more than you can help. And certainly, do not brag. If you are sweet and nice to people beneath you, people will generally like you. If you treat people beneath you as if they have no feelings, the quality people will avoid you. Be ready to field a response. This can range from a pathetic plea for mercy; to a nasty, mean, angry letter; to a truly hurtful action; to flowers and a renewed attempt to seduce you; to a hand-held grenade launched in your direction; to some subterfuge and game playing. In the case of potential violence, alert others. Otherwise, ignore everything. If it persists for long, then ask the person polite to stop it. Remember that it is not because you are perfect. If you have never received a Dear-John letter, it is not because you don't have faults. It is probably because your loved ones accepted them as a part of you and realized that the good in you outweighed the bad. If you can't see your own faults, then you could be a narcissist or just don't want to acknowledge the possibility. Either you will get a Dear-Jane letter one day or you are so insecure you are choosing people you are sure will never dump you. Be sure to factor in the feelings you would feel the moment you realized you had gotten a Dear-Jane letter. 
Decide whether you want to a) hurt the other person a lot, or b) cause them minimal pain. If you choose a), the letter is easy. Eliminate all criticism of the person if you want to soften the blow. Be clear that you want to terminate the relationship. Keep the letter short. Make sure you have privacy and that the person has an escape route if you choose to do it in person. Provide one simple explanation about why it is not working and make it about you, not them. Remind the person that your rejection is not a reflection on them, even though it obviously is. Mention to the person in what ways they have contributed to you or something positive you learnt from them that you will carry forward the rest of your life. Wish them luck in future relationships and let them know you still care about them as a person and hope that your friendship can survive. Do not talk to others about the break up any more than you can help. Be ready to field a response. Remember that it is not because you are perfect. 