According to the Data Modeling unit on Trailhead, some of the key actions that should be done when data modeling in Salesforce are identifying data elements, mapping legacy data, and implementing legacy data model. The unit states that "Before you start creating objects and fields in Salesforce, you need to identify the data elements that you want to store and work with. ... Next, you need to map your legacy data to Salesforce objects and fields. ... Finally, you need to implement your data model in Salesforce by creating custom objects and fields using declarative tools or Metadata API." Therefore, these are the correct actions for migrating a legacy client server app to Salesforce.