I shall be doing this on Monday. 😀
Last time I did it, it said something along the lines: This letter constitutes 3* months' notice of my resignation as an employee of [NAME OF EMPLOYER]. Accordingly, I calculate my last working day, subject to agreement in respect of accrued holidays for the period, to be [DATE].
You can then go on about how you're looking forward to your new challenge, etc., but unless the letter is for your boss (in which case, get off the fence or stay right on it), it's just for HR recortds, so doesn't need to have any more detail.
Save the slaggging of key management imbeciles for your leaving drinks. 🙂
*I'm now up to 4 months' notice. I reckon I'll do 4 weeks.