letting them know that I am stepping down. I'm not exactly sure how to do this. Do I just verbally say it, or write a letter?
Be very very VERY careful.
Be prepared for multiple, insistent and outrageously nosy questions and unrelenting pressure to recant your resignation.
They will also undoubtedly offer to "lighten your load for a time" so that you can continue to serve as an MS.
Finally, even if you manage to stand your ground, it is very likely that they won't accept your resignation until the CO's next visit.
Any hint of "apostate" thoughts during those multiple, insistent, and outrageously nosy interrogation sessions will be met with quick judicial action.
It's fine to step down, but make sure you have a plan and stick to it.
Practice deflecting questions and saying "I'd really rather not talk about it right now." Be ready to just shut up before saying too much.