Your school probably has its own guidelines for writing this assessment, so do check with your Dean and in your Division Office for a template you can use. Since you are teaching at a school that uses narrative evaluations instead of traditional grades, you may well have been encouraged to have your students create portfolios encapsulating the best of the work they have done for you this semester, along with their own self-assessment essays. Gather these materials.
While reading through each student's portfolio, look for evidence of creative learning, independent decision-making skills, problem-solving abilities, evidence that he has learned how to learn, evidence he can work collaboratively, and evidence he has acquired the ability to manage himself in his chosen field. Address each of these criteria in your narrative evaluation. Be as precise and specific as possible.
Having read your student's self-assessment and written your detailed narrative, it is now time to decide what sort of academic credit your student will receive for the work he has done in your class. Most schools that use this assessment method mark student work as either satisfactory, unsatisfactory or incomplete. Make the final judgment fairly and honestly. Take pride in teaching at an academic institution that is making strides in eliminating grade-grubbing and grade-inflation.