Perform a self-assessment by documenting the status of conformance with each major standard. This can be done in a two hour staff meeting, using the Self-Assessment of Conformance template and the 2017 Standards. Today this would be done using the new Global Internal Audit Standards. My materials are not updated for those new Standards.
Prepare a draft Self-Assessment Report.
Invite a CAE-type from another organization to visit for a day:
Explain your Self-Assessment of Conformance
Allow them to review any documents you reference, and read several audit reports, to understand your opinion of conformance. Maybe also phone-interview those to whom IA functionally and administratively reports.
Complete a Validation Statement and the Self-Assessment Report
Best if done in a three organization format: A review B, B review C, C review A.