When writing for a letter to ask for a previous job back, it is necessary to explain why the current isn't a right fit and explain the reasons for wanting to return to the previous job. The letter should be convincing and clearly show the previous employer that the employee has strengths that would benefit the company and is worth rehiring. If the person had been terminated from the previous job, then this will make it more difficult to get it back.
Before writing the letter, do some research to find out if the previous position has been filled and if any changes have occurred in the company. It is also a good idea to connect with previous coworkers who may have information like this.
When writing the letter, explain what qualities of the new job are not working out. It is also a good time to show sincere regret for leaving the previous job and remind the recipient of the accomplishments and performance the former employee had at the job before leaving. A main goal is to show the previous company that it would make good business sense for the company to rehire their former worker. If the person left to pursue a higher position, it may also be helpful to suggest returning to a higher role, especially if the old job has been filled.