The custom fields can be found in Reviews by clicking the Manage fields button in the top right corner.
In the Manage fields menu, you can create, edit, and delete custom fields you've created. Click on the Create field button to create a new one.
In the field creation view you can name the field, which also automatically generates an ID (Code) for it. In the drop-down menu you can select the type of the field, which can be a text field or a text area (this means basically just the amount of text). Lastly, you can select the categories the field is applied to.
When you're ready, just click Create new to finish the setup.
Now that you have a custom field, you can add it to reviews during the import process by clicking the Manage fields button on the import view.
Here you can see all the data the imported review includes. On each data type, you can see a drop-down menu, in which you can view and select the custom fields you would like. Rememeber to click Save when you're done!
Finally, the custom fields can be seen on the imported review card, right next to the name of the reviewer.