I wrote up something that I was going to send my mom to come out to her, but ended up doing it face to face. I think it turned out better that way (for me, at least). She's even going to my second therapy appointment tomorrow and agreed to help cover the cost, but she's a different case. Always been very supportive of the alphabet community as a whole.
What I tried to focus on in my letter was the things from my past that made me realize it, the trouble and pain it has caused me, and the positive things about why I feel like I need to tell her. I find that communicating things about dysphoria (if that is your case), though not fun to read or talk about it may be, draws light to the seriousness of this decision. However you put it, don't come off too lightly. Being trans, in my opinion, is the biggest decision you could make in your life, short of going all the way with transition.
I wish you the best with writing, sorry I can't be of more help. I could share what I wrote in a private message if you'd like. It's even more personal than my intro post, if that's possible so I'd rather not post it. For me, just writing it down helped me gather my thoughts for the actual face-to-face and talking about the things I wrote out loud to my therapist gave me the extra push to vocalize it. Good luck
