During an ISO 9001:2015 internal audit, best practices include ensuring auditors are competent and impartial, preparing a detailed audit plan, and using a checklist that covers all standard requirements. Auditors should focus on complex processes and areas with a high probability of nonconformities. Nonconformities should be controlled and corrected, with root causes identified and corrective actions implemented to prevent recurrence. The effectiveness of these actions should be reviewed, and documentation updated as necessary.