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 to prevent recurrence. Corrective actions should be implemented, their effectiveness reviewed, and documentation updated as necessary.