1. Create Item Options Group or utilize existing. 

Create New Item Type:

Once the new Item Option has been created, edit the Product you would like to add this variant option and select "Edit Variants" from the main editor screen.

Select the option type and begin adding in the option values one at a time.  

This will build out the lower half of the screen where you can add the option prices and optional variant-stock-numbers.  

Press save when completed.