How to write a letter of resignation under duress?

I want to wrtie one but I need some guidance to write this letter. I am not that great with my verbage. Thank you
Answer:    Dear (name of boss/manager/supervisor),

This is to inform you that effective two weeks from the date of this letter, I resign my position from (company name).

I have enjoyed my time with the company and I would like to thank you for the opportunity to learn and develop my skills.

Sincerely,
(Your name)

That's the letter I used when I quit my job two months ago.
Last year I was given the choice to resign or be fired, and when I chose to resign the boss asked me to write a letter of resignation. Because I wanted to leave under the best possible circumstances, I thanked her for the opportunity to work for the business and told her that I was leaving because I wanted to seek other career opportunities. I left the letter open-ended as to whether my resignation was effective immediately or whether I had given advanced notice, as I was forced to leave the position immediately, therefore, did not have the opportunity to give notice as I would have under normal circumstances.
If you're under duress think carefully about the wisdom of writing this letter at all. This is a document that is going to be used to prove voluntary departure rather than termination, which will compromise you if you intend to collect unemployment, or if you intend to initiate legal action.
With that said, if you still need to write this letter, be sure to site why you're leaving. Say something like "As Mr. so and so has asked me to resign, I am providing this letter of resignation at his request" Be as specific about the details as you can to make it obvious that your departure isn't really your idea. You may even use the word "duress" to make sure the point is taken.
If it was me, I would certainly wait and force the company to offer me severance or fire me.