Do you have any advice for a coming out letter to be delivered in person?

I think it would be best to start by building up to it-- you know, certain things, like eye color, etc, are not choices. Mention that you have gay friends, things that should've been clues to her. Tell her around the middle, but maybe not "I'm gay" but "I date men" because often the word itself carries a negative connotation. Finally, wrap it up by explaining that you're still her son and nothing will ever change.