How will you rate your employee's performance?

Set a performance standard and highlight key areas of employment via a performance agreement and job description.Both supervisor and your employee agree on above.Perform quarterly assesments based on the agreed standard of performance ,highlight area's of excellent performance and draw attention to area's of weakness .Design a training schedule to address area's of weakness and turn them into strengths.

By employee's performance data of previous year monitored in defined KPI, target, goal,and frequency thru thru formal interview to EE.
On time constructive feedback and coaching is required if the employees performance is slipping on agreed goal.
From that interview, employee and employer can have an agreement to set another of goal, objectives, and target for the present year.
Employee must given a reward, token, commendation or any sort on his good performance

SOPS or standard operating procedures can be used to measure employee performance however this would not be adequate without regular or periodical staff appraisals where the supervisor would highlight staff strong points and identify weaknesses for the purpose of training/discipline.
Furthermore the presence of the supervisor with that super vision will enable him/her know the best performers and the slow goers.
Additionally delegating some specific tasks to staff helps to develop them and know their capabilities.