Resignation etiquette for non-profit?

This non-profit job is my first professional job after college so I am not sure how the whole "resignation" thing works. I have the assistant director of development who supervises me and pretty much the only "manager" I talk to and gives me tasks while technically the director of development is my boss-boss. I rarely speak with the latter and have not told her but I am sure my assistant director have already given her the news as I already told my assistant director that I have found a new job last week before I left for vacation and will be officially sending in my resignation letter tomorrow once I get back. Since we all work from home, should I just type out the letter and send it as a document to both my assistant director and director of development? Should I tag HR in the email too?

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