Prepare for It. In this case, all you can do is politely deliver your resignation, then strive to make the transition as easy as possible for your team. Your manager may not have the ideal reaction to your news__and your workplace may not be the most pleasant environment for the next two weeks__but to avoid burning bridges, it's best to work through it with a positive attitude.
Breaking up is hard to do, but pouring your heart out on paper is rather awkward. There is really no need for any other information in your resignation letter (including a reason __ which is completely optional). No effusive template and formatting effort is required. This is not a letter that is going to be hanging on anyone's wall, and it probably won't even be filed away!