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

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.