Custom Fields are defined and created directly in the product.
Go to your BigCommerce account. On the left side menu, search for "Products" > "View" > and select the product to which you want to add custom fields.
Look for the option "custom fields" and add as many fields as you need (with their name and associated value). Remember to click "Save" to save the changes.
Finally, from the doofinder app, activate the option to "Index Custom Fields".