In case you have an existing Magento system that you are linking to Business Central, chances are that you want to import the existing data. Importing data can be done multiple ways. You can use standard Business Central Rapid Start functionality and additionally there are the Commerce 365 for Magento - Excel data imports. Click here for more information. But if want to import from an existing Magento, you probably want to import directly by using the Magento REST API's.

Click on Administration > Data Import (Magento) to get to the following screen. 

The menus on this page are divided into Attributes, Categories, Items and Images. If you work your way through these menus from the left to right. From attributes to images. You should be able to completely import your existing Magento item catalog. 

This features is, at first, designed to work with a non customized Magento. If your existing Magento catalog or API's contain many customizations it might be possible that you cannot import all data without making changes to either the data or the data processing workflow. 

Click on a number in the first column to open the complete log for each data import task. Here you will see exactly which data was imported and where there were errors. 

When importing items you will see a second dialog asking you whether you want to create items that do not exist yet. When enabled this will create both the standard BC item (table 27) and the Magento item. When not enabled, this process will only import Magento item data for items that already exist in table 27.