Every situation is different. It is also hard to predict how people may react. People get surprised all the time from both positive and negative perspectives. But, from what I’ve seen, people seem to be pleasantly surprised more often than not.
Anyway, I can only pass along my thoughts, but you are the one who must ultimately decide.
It is always important to come out on your terms as it is hard to recover if you are outed. The problem is that if you are outed, you have no control over what is said. It can be driven by misinformation, purposeful misinformation and prejudice. Further, it works both ways. The negative ones will be pissed because you are the cisgender heterosexual person that they thought. The positive ones may be upset because, they feel, you didn’t trust them enough to tell the truth. It can be a minefield either way.
As is said, the best defense in an offense. In this case, it means to take the initiative. Give the process some conscious thought as to what you need to say. Remember that you are telling YOUR story YOUR way. Taking notes to capture your thoughts may be helpful, but, reading from a script may come off as impersonal.
My story is this:
I came out October 10, 2015. I was still working then and was a member of our LGBT employee affinity group. Every October we brought in LGBT comedians for a free event that was open to the public. Previously we had Suzanne Westenhoefer twice, Marga Gomez, Alec Mapa and in 2015 we brought in Ian Harvie, out first transgender performer. To me, it was a golden opportunity. I had been out in the public many times before, but all an hour to an hour and a half away from my hometown. I volunteered to be Mistress of Ceremonies as DeeAnn and the group accepted my offer. The result was that I came out in front of about 130 people, including some co-workers. In the course of that month, I discussed the event during 2 other gatherings. I was not dressed, but I closed with a photo from the event with Ian Harvie and me. The total was about 210 people for the month, which included my daughter and son, 7 or 8 close friends and my department manager at the time. Sometimes, for whatever reason, my thought process is Go Big or Go Home. While it suited me at the time, it is not a methodology that I would recommend to others. What is important is a way that makes sense and feels appropriate for each individual.
On final point about the “How”. Personally, I would only do a letter or an E-Mail as a last resort. I did talked to my kids on the phone (individually), but that was because they both live at the other end of the country. I had no time to do otherwise. To my mind, there is a parallel. What would you think about being told that your significant other was breaking up with you by E-Mail or being told that you were laid off from your job by E-Mail? My interpretation, and purely my own, is that it would feel disrespectful and impersonal. Conscious thought is your friend!