Brainstorm your thoughts and ideas on a piece of paper. Include reasons that you want to split up, and ways to write them out as nicely as possible. Jot down some nice things you can add about your ex -- include good qualities you think he possesses. Include the role you see that person playing in your life in the future and why or why not you feel he should be in your life.
Hand-write the breakup letter. This adds a more personal touch and shows that you took the time to write it out. Make the letter more formal by addressing it to her and dating the top. Begin your letter by explaining that you don't think you should date any longer. Be blunt but gentle and to the point. You don't want to give any mixed messages that might confuse her.
Establish that your romantic relationship is over, and continue by writing out the reasons why you feel you can no longer date. Again, be as gentle as possible here. Avoid using terms that involve blatant rejection, such as "dump," "I don't want you" or "You're not my type." Consider softer language such as "It's not meant to be," "We'll be better as friends" or similar. Continue by writing about any positive qualities she has and how much you enjoyed your time together.
Finish the letter with a description of how see your future with this person. Be honest. If you don't want him in your life at all, tell him and explain why. If you want to remain friends, let him know and give him the option to accept those terms. Don't try to force him into another type of relationship; give him some time to think about it and leave the option open for the future.