Identify Fields in Related Records in Workflow

I am having trouble wrapping my head around how to identify fields in related records within a workflow.

I am using a training DB.

I have a form called Furniture Request form

I created a workflow and one of the states is sending an email to the customer.

What I am having difficulty understanding is how to identify the fields that are NOT located on the Furniture Request Form.  In other words, the customer phone number and balance and the employee email and phone.

I understand how to set preferences to show internal ID's.

I understand that I get the field names from the records, in this case the Customer Record and the Employee Record; which show Customer Record - Field ID = phone and customer Record - Field ID = Balance.  Employee Record Field ID = email and Employee Record Field ID = Phone