Simply replace the unfilled areas such as "" and "[date]" with your information.
To Whom it May Concern:
[Mr./Ms./Mrs. & last name] joined our team on [date that they joined company]. Based primarily on [his/her] initiative, skill sets and work ethic, [employee first name] has become a valuable resource.
[His/Her] attitude was always positive and [his/her] productivity always expectations. To say that [he/she] utilized [his/her] skill sets to develop or improve procedures would be an understatement; [he/she] excelled at the task.
While I am unable to fully expand on all the positives associated with [employee first name] in a letter such as this, I can say that [he/she] was considered a valuable asset and that [his/her] contribution will be missed.
I would be pleased to recommend [him/her] for any position where [his/her] skill sets can be utilized.