You must specify where to show the fields, else, it will duplicate and show the custom fields multiple times, all over your pages.
Besides how to use the “Display on” options to let the plugin know you will show custom field on the product page or checkout page, you need to specify the position you want to show.
If break the process into 2 steps, you will have:
- “Display on” options: select what page to show
- Rules options: select where to show on that page