ISO provides “How to write standards” for new editors on Youtube, which is linked at the bottom of this page. It is based on a fictitious standard, the “Rice model”; as all ISO and IEC standards are, it complies with ISO/IEC Directive Part 2, which is the process document you must comply with. It provides all the rules that an ISO editor has to follow while editing ISO as well as IEC documents. Directives Part 2, alongside other rules and policies documents are found at https://www.iso.org/directives-and-policies.html.
ISO also provides a set of more descriptive documents to help editors. If you go to the “Drafting standards” page, you will find various documents that will help you. Among them, the “Model document of an International Standard – Rice model” is a fictitious standard that explains as many examples as possible of a real standard.