A resignation letter has one job: to put on record that you are leaving and when. Everything else in it exists to protect the relationship on your way out. That means the structure is short and fixed, and you do not need to be a strong writer to get it right.
Every resignation letter contains four things, in this order:
Open by resigning
A direct opening sentence stating that you are resigning from your job title at the company. No preamble, no build up.Name your last day
Your last working day, written as an actual date. Do not write "in two weeks" and leave your manager to do the arithmetic.Say one real thank you
A short, genuine thank you. One or two sentences about something real is worth far more than a paragraph of compliments that could apply to any employer.Offer the handover
An offer to help with the handover. Naming what you will actually do, such as documenting your work or training a replacement, is what makes this line believable.
Then sign off. Four short paragraphs is the ceiling, not the target. A long resignation letter reads as defensive, and nobody in HR will thank you for it.