The key is frequent communication during the year so there are no surprises at the end of the year. At a minimum, the supervisor should hold quarterly if not monthly feedback sessions. I also recomend 1) using a report card since that makes things even clearer; and 2) for multi-encumbered jobs, posting everyone’s performance (although not by name) so that each employee has a sense of context.
At the end of the year, be honest with the employee and get his perspective. Treat him the same way you would want to be treated.
Here are links to two articles I have written on performance appraisal in government:
I have also written a book on managing performance in government that you may find to be helpful: