how do I write the letter of resignation?

I would make a distinction between retirement and resignation from a legal standpoint.
As I understand it, depending on local law, a notice of retirement might more usually be from an employer to an employee who has reached retirement age, though one would hope retirement would be by mutual consent.

So far as resignation is concerned I have previously used the following as formal statements:

'I hereby give notice of of my intention to resign my post as as required by my contract with '

Example

'I hereby give notice of one month of my intention to resign my post as Road Runner, as required by my contract with Acme industries.'