However, if you are seeking a letter of recommendation in order to help get you employment somewhere else, then you will need to write a polite letter to your ex-boss explaining exactly what the new job is and what you would like them to write in the letter.
If you are writing a letter to seek a job opportunity with your ex-boss again, then you might want to set out a letter like this:
Dear Mr/Mrs Smith,
I am writing to you to ask about any potential vacancies and job opportunities that might be available at your company at present or in the near future. I am currently seeking new employment and I would be incredibly enthusiastic about working at [name of the company] again as I enjoyed my time there in the past a great deal.
As I hope you can remember, I have a very strong work ethic and maintain the utmost standards of professionalism in all of my work. My list of achievements when I last worked at the company included [details of achievements].
Since leaving your employment last I have worked as [detail your other jobs here] and I believe these experiences have enhanced my abilities considerably.
I would be grateful for any opportunities you might be able to offer.