How do I construct my resignation letter?

Hello, Everyone; I need some advice on constructing my resignation letter.
I currently work at a non-profit as a coordinator and began in this role in February 2022. Honestly, the job was incredibly toxic, with the executive director causing me incredible stress as the only coordinator in the office. We do not have a human resource administrator, as the solely individual working at the office is the executive director, myself and an administrative assistant hired last week, as the other one resigned.

I recently received a government internship offer that I'm taking, which will begin at the end of the month.

I plan on emailing my resignation this week and would appreciate advice on constructing the letter.

Thank you!

How do I construct my resignation letter?
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