The next steps depend on the Data Source, if you select QuickBooks Online the next step will be to login to your QuickBooks Online subscription. In the case that an extension does not exist for your previous finance system then the data source that can be used is Excel. Business data from the old finance system can be exported into excel and then imported into Business Central.
In the next step you can download the excel template. Additionally, you can Press the Settings button and define Templates with predefine values for the Customers, Vendors and Items.