Dear Mr./Ms./Mrs. (name),
First Paragraph: This paragraph contains only the motive of your retirement including your current designation, name of your organization and date of your retirement. It should be clearly written without any error to avoid any confusion in future.
Second paragraph: In this paragraph mention about your tenure in which you can explain your experience with the company, colleagues and management. Write about your memorable meetings and wonderful achievements. This paragraph communicates thanks to the employer for his/ her appreciation and do not forget to acknowledge those people who have helped you during your tenure.
Last Paragraph: It states your even eagerness for kind transition. Give an idea about your future plans after retirement, if you want to work somewhere. Always show willingness to be there, if any case company needs you after retirement.
Wish the organization all the success and all the best to successor.
Close the letter with thanks and regards.