I am fully qualified internal ISO auditor. You need in place procedures for everything your business does and i mean everything. A paper trail that always can be traced. You also need an integrated management system manual.
TBH a helluva lot of work which costs money to maintain. Audits carrie dout by someone qualified at certain times and once per year a visit from an external source to go over everything.
you will need procedures on how you fill yer tank, chage resins, H&S items. You will also need procedures i place regarding how invoices are worked from your end and also items regarding what happens in payment not received.
You can also have an approved list of contractors you use on your ISO list.
don't bother with it mate.