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.
![add a custom field](/images/big-custom-fields.png)
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.
![add a custom field](/images/big-custom-fields-1.png)
Finally, from the doofinder app, activate the option to "Index Custom Fields".
![activate custom field indexation](/images/big-custom-fields-2.png)