Oh. I guess I kind of overlooked that last sentence.
I'm very bad socially too, so I always go with at least something written. I've only done it twice now, to 3 people, although I had planned to do it in person for the last two.
The first time I did it, I wrote a very long email detailing my entire life that they didn't know about. This was to my mother. Don't do it this way. It's too much information at once. It's fine to follow-up with something like this, but it shouldn't be the initial statement. It took several additional in person discussions with her to get her to be okay with it and in the meantime, she ended up depressed thinking it was her fault.
The second time (last night), I also did an email, but it was mainly just an overview with some extra stuff added to lighten the mood. It's really important to customize for the target audience. Put yourself in their shoes. As you're writing what you want to say, expand on any questions that come to mind because if you're in their shoes, they'll probably come to their mind too. For instance I learned with my mother that her immediate reactions were that I was gay and it was somehow her fault, so I addressed the gay question with my friend. People also tend to think it has something to do with sex (at least for MtFs... not sure if the same applies for FtMs), so I addressed that. This particular friend asked if guys also thought that it would be easier to be a woman (because she's thought the opposite) on Facebook, so I expect her to think that about what I typed and addressed that as well.
If you're having trouble thinking of things people may counter with, there's a whole thread on the subject here:
https://www.susans.org/forums/index.php/topic,124007.msg971056.html#msg971056Always do a follow-up to address any questions and do that in person if possible. It's pretty scary, but I've had good luck with it so far. I spoke with the friends I emailed last night, this morning, so I have 3 under my belt so far.
When I planned to do it in person, I had written up a short statement that I could read from. For one thing it made sure I didn't forget anything and for another, it channeled my mind down the path I wanted to follow. Then I had a longer, more detailed letter to leave with them (this was a therapist suggestion).