You start with any documentation they already have (usually done for insurance or legal reasons) and start mapping much in the same way you would map data flow in a program.

Other sources are any existing ERP or other business process system.

Paperwork of any kind (invoices, quotes, POs, accounting) can also be used.

Even if a business thinks they are a flat org and have few processes, I guarantee there are ad hoc managers/leaders along with shadow processes.