How do you prepare for a formal appraisal when you are the person being evaluated?

You don't say whether you are a student or employee, but in either case, it would make sense to take evidence of how you had met or surpassed your performance goals in the previous semester or year.

You might think ahead of time about any weaknesses you see in yourself, and how you plan to address strengthening them in the near future.

--For instance, if you are a student and your writing assignments are less than stellar, you could say that you are meeting with a writing coach/mentor to show first drafts and using their advice to revise assignments before turning them in.

--If you are an employee, if you are having trouble meeting benchmarks, you could say you are taking night classes to strengthen your computer skills, or you have joined Toast Masters to enhance your presentation skills or whatever.

--Finally, you could identify areas in which you would like to increase your expertise/responsibilities and ask for the opportunity to follow that path.

Your goal is to show the person conducting your evaluation that you are self-aware and devoted to improvement.

Review your performance (if a performance review or whatever is being reviewed), inventory any strengths/weaknesses, accomplishments, challenges, etc., and identify any questions about how to maintain or improve your performance (or whatever is being evaluated).