And here's my suggestion for an intro of the letter:
"To whom it may concern:
I’ve been a baptized Jehovah’s Witness for over 99 years. I’m currently an active / an inactive Publisher / Regular Pioneer / Ministerial Servant / Elder / Circuit Overseer / Bethelite, assigned to a congregation in state / country.
This letter is intended to let you know that I no longer wish to be identified as one of Jehovah’s Witnesses, and therefore I wish to be dissociated from this congregation and its legal entity, the Watchtower Bible and Tract Society, as well as any of its branches and brands. I can’t tolerate any longer the embarrassment and the mental suffering that comes from being associated with this harmful cult, and these are my reasons: "
(The BOLD letters are the items that have to be personalized, even for those who wish to remain anonymous.)