My folks are divorced. I told my mom in person, in public. I was positive that she would be accepting, but I didn't want her to ask anything too personal about it. But then I remembered that my mom has no boundaries, so I ended up answering really personal questions over pizza. No issues there.
I was worried about my dad and stepmom. They are both conservative and religious. I was a lesbian prior to coming out, and they had never been very thrilled with that but had finally gotten okay with just letting it be what it was. I wasn't sure they would understand, agree, or support me being trans and wanting to transition. So I did as much as I could before hand to make sure that they would see how serious I was about it - I talked to multiple therapists, I made an appointment to start T, I mapped out my intended medical transition and made a list of online resources. Then I wrote them a letter explaining everything. My intention was to go over, hang out, give them the letter, and then leave. I usually go over there to visit on weekends, so I figured I'd give them time to process and if they didn't want me to come back again, they would have a week to let me know.
But I got crazy nervous about it about midway through the visit, and my dad could tell that something was wrong. He asked me what was up, I started crying, and we had a conversation about everything. He was surprisingly supportive. Told me he didn't understand it, but would do whatever I needed him to do to make it happen. Stepmom agreed.
I think writing a letter is a good move, but I think it should be more for yourself, in order to gather your thoughts and lay out how to present it to other people in a way that will make sense. I don't know that I would just mail something off and wait.