To ensure the data is loading into the correct fields in Financial Services Cloud, the consultant should utilize a field mapping file with the external system allocation and the corresponding field in Salesforce. A field mapping file is a file that specifies how fields from an external system map to fields in Salesforce. By using a field mapping file, the consultant can ensure that the data from the new custom object is imported into the correct fields in Financial Services Cloud and avoid any data loss or inconsistency.