After you know both why you are evaluating the employee and who will be doing the evaluation you can then design a tool that will measure the items that you want to evaluate.
There are generally three different kinds of focuses that an evaluation tool can have: a trait focus, a behavior focus, or a result focus.
Trait Focused Performance Appraisal System
In a trait focused performance appraisal system, specific traits like dependability, helpfulness, punctuality, etc. are evaluated. This particular appraisal system is often used in customer service evaluations, however, they have many drawbacks. First they are extremely subjective to personal biasing and circumstantial situations. Secondly the reliability and accuracy of these types of evaluations are relatively low. Because of these drawbacks trait focused performance appraisal systems should therefore only be used as a supplement to a more reliable form of an evaluation tool.