Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Hiring Manager],
My name is [Name] and I am the [title] of [Company]. [Employee Name] was employed here from [date] to [date] in the position of [position], [His/Her] general duties included [duties], [duties], and [duties]. Overall, [Employee Name] was a less than ideal employee.
While [Employee Name] has the ability to perform the basic job functions, [he/she] typically [completed/did not complete] most of his assignments [on time/ late]. However, they were typically [adjective] and [adjective]. [He/She] [was/was not] open to constructive criticism and [did/did not] show improvement. [Employee Name]'s overall attitude in the workplace was [adjective].
[Employee Name] may have become a better [position] since [his/her] time with us. However, I would hesitate before I would recommend [Employee Name] for a similar position.
Please contact me if you have any further questions.