https://www.slab.org.uk/faqs/step-1-get-ready-to-self-assess-against-the-organisational-standards/
https://www.slab.org.uk/faqs/step-1-get-ready-to-self-assess-against-the-organisational-standards/
Watch the Introduction to the audit function presentation here.
Watch the guidance slides for each of the individual Organisational Standards.
Read the Self Assessment Guidance to help you identify what evidence the auditor will be looking for when you are audited.