Dear Mr./Ms. Manager?s Name,
Please accept this letter as to formally announce my retirement as Position Title to become effective as of Date.
I am looking forward to enjoying my retirement. I will be happy to help with any transition during my remaining time here. Please feel free to get in touch with me after I leave if I can be of any help.
I have greatly enjoyed my time employed here and offer best wishes to my friends at Company Name.
cc: name of H/R supervisor