You can’t. Don’t try.
Consider what the goal of a performance appraisal discussion is— and what it’s not. The goal is not to gain agreement. If you gain agreement, that’s fine, but it’s unlikely if the appraiser has evaluated the individual’s performance against tough-minded, demanding standards. In fact, the lower the appraisal rating, the less likely the individual is to agree with the assessment. That’s okay.
The goal of the performance appraisal discussion is to gain understanding, not agreement. Whether or not the individual agrees with the assessment, or the factors that were used in making it, or the standards that the appraiser expects the individual to meet, the manager’s job is to get the individual to understand the reason that his performance was rated the way it was.
Gaining agreement is nice; gaining understanding is mandatory.