In the Admin section, under "Item Attributes" select the Item Type you would like to add Item Attributes too from the list.

Clicking on the Item Type will open up the details for that Item Type and allow you to add Attributes.

Users can choose to add new Attributes by click the '+' icon at the top right and entering the Attribute name they would like added in the input field to the left of the '+' button.

If attributes are already added, you can select attributes to be used for products and product variants by selecting the 'Used for Variants' checkbox above the attribute list.

For the example above, we have selected Metal Type to be used as the main attribute level for the Necklace Item Type.

Note: An attribute will need to be selected for every Item Type that will have product variants.