After you select your item category and options, you will see this section after the description boxes.
After you select ‘yes’ more options will appear. You can select from the variations you used in your listing or check the box to add a custom variation.
You will need to add a name for your custom variation. You will add this in the text box (1) and then save it before you can add options (2).
Add options for your custom variation in the text box (1) and then save it (2). After you have added and saved all, set your option by clicking the ‘set variation option’ button (3).
The variation will open like the image below. You can confirm the quantity and add a custom price or image if applicable and then save it.
Please note that you will need to add the original option as a variation.