I gather you are referring to an ISO 17025 internal Audit?
In that case, the person performing the audit needs to know what the Scope of work is (what tests or calibrations are accredited, is sampling included etc) and what processes are involved. Thereafter the audit criteria (the requirements being assessed) need to be documented. The simplest way to achieve this is to use a suitable checklist. This could be your own checklist based on the ISO 17025, however the most straightforward approach is to use the accreditation bodies checklist (which they use for their assessments) or the Advisera toolkit. Witnessing and vertical assessment should also be performed.
For further information see the article ISO 17025 technical internal audit: The basics
The Five Internal Audit Procedure appendices Internal Audit Program, Internal Audit Checklist, Audit Nonconformity Report, Internal Audit Process Checklist and Internal Audit Report available separately from the procedure link above; or included in the toolkit at https://advisera.com/17025academy/iso-17025-documentation-toolkit/