Write down the owner of each field
List the fields the integration will touch and assign one owning system to each. Company name and sales stage may belong to the CRM. Billing contact, payment terms and invoice status may belong to the accounting system. Contract dates may belong to a contract repository. The owner is where the value is created and corrected; other systems receive copies.
A field without an owner is a warning sign. If two teams expect to edit the same value, decide whether one system becomes read-only for that field, whether changes route through a request, or whether the field should be split. "Billing address" and "shipping address" are often two fields that were stored as one.
I would also record the business meaning of each field. Two systems may both have a "status" field that means different things. Mapping them directly creates a record that looks consistent and is wrong.