Auditing

If you need advice about auditing to ISO 45001 then a key standard you should refer to is ISO 19011:2018 Guidelines for management systems.

Also note that ISO 17021-10 Conformity assessment -- Requirements for bodies providing audit and certification of occupational health and safety management systems was published in March 2018; it highlights the competence requirements for auditing and certification of OH&S management

Certification

The International Accreditation Forum, inc. (IAF) facilitates trade and supports regulators by operating a worldwide mutual recognition arrangement among Accreditation Bodies (AB's) in order that results issued by  Certification Bodies (CB's), accredited by IAF members are accepted globally. 

The IAF's requirements for ISO 45001 certification activities are defined within their document MD22 Application of ISO/IEC 17021-1 for the Certification of Occupational Health and Safety Management Systems. Its additional requirements are something those seeking certification should consider - including whom to consider when addressing interested parties and those persons who must be interviewed.