When writing a self-appraisal of work performance, focus on personal achievements and contributions made to the overall success of the staff team or unit. If addressing mistakes or areas of weakness, talk about lessons learned or areas for future growth rather than describing failures or specific errors in detail.
Self-appraisals call for succinct, descriptive statements, not detailed descriptions and excessive use of superlatives. The purpose is to help management recall one's accomplishments and the impact of one's work on the business overall.
Avoid criticizing or blaming others when writing a self-appraisal and ask specifically for what you need to continue to grow in your position. When describing what you need, such as training or support, do so in a developmental context rather than as an excuse for mistakes. "Training on outcome measurements would be very helpful" is better than "It didn't work out as planned because the supervisor wasn't available and the folks collecting data were a mess." When addressing skills in a self-appraisal, emphasize how personal skills contributed to successful outcomes rather than how your skills are underutilized.
Meaningful, useful self-appraisals require the attention of both manager and employee. The manager needs to clearly define the purpose of the self-appraisal and provide questions to elicit information upon which both can build. The employee needs to thoughtfully approach the exercise and request clarification as needed.