Enter the title of your custom field in the Field Name box, select what type of field you'd like it to be, and click Create.
Once your custom fields are created, you can update the fields in your contacts individually or include the fields' information as you perform a mass import of contacts. For the latter, simply add the field names to the provided Address Book CSV template as headers of additional columns.
To edit an existing Custom Field, head back to the Settings tab in your Address Book, find the field you'd like to adjust, make your changes, and click the Update button.
To delete an existing Custom Field, click the Destroy button next to the field you'd like to remove. Please note that deleting a field will remove it from all Address Book contacts and all data related to that field will be lost.