Anyone that has worked on the backend of Magento 1 importing products knows that Magento 2 has rewritten the book on how you import in custom product attributes in the Product CSV file. Where Magento 1 used to allow a columnar approach where you could do 1 column for each piece of information (see below)
|Custom Product Attribute 1
| Custom Product Attribute 2
|Custom Product Attribute 3
|Attribute 1 Value 1
|Attribute 2 Value1
|Attribute 3 Value1
|Attribute 1 Value 2
|Attribute 2 Value2
|Attribute 3 Value2
Magento 2 does not allow that individual column (per attribute) approach, but rather a single area called "additional_attributes" that is used for all your custom Magento 2 product attributes.
Now, if you have a web developer on staff this won't be an issue because developers can write code to bring together your old format with the new format. However, if you are an owner-operator or don't have a web developer available it may be difficult or time consuming for you to match the new format of the Additional Attributes field.
I have created a template that does all the heavy lifting for you and all you have to do is update the Google Sheet with your own store's information.
To make your copy of my sheet go to File and click Make A Copy. You will need your own Google account to do this.
When you are done with that additional attributes field copy the row and PASTE SPECIAL into your Magento Product Import CSV. Sample Product CSV here.