To assign item extras to items, select "Inventory" and then "Item Extras" from the main navigation. Hover over the item extra that will be assigned to items and select "Manage Item Extra".
On the settings page, select the "Assigned Items" tab.
From the item extras page, hover over each grid item of the items that will be assigned to the selected item extra and hit the "Assign Item" button.
Note: Item extras can be assigned to multiple items at once.