On Extras click Add new Extra:

• Is Active? / Show Ratings? toggles.
• Main image plus up to three gallery images (drag to reorder).
• Display Name (required) and Internal Name (Only for Admin).
• Price (required) and Cost of Goods Sold (for margin reporting).
• Sorting order - higher numbers are listed first on the storefront.
• Product Categories - multi-select of your product categories (used for filters and Display by category).
• Weight / Height / Width / Length - shipping dimensions.
• Track Stock? with Stock Qty and Low Stock Alert.
• Description (up to 2000 characters).

Click Save; the page shows a Purchased counter and the star rating for existing extras.