When to use it
Populate the supporter data related to your organization's custom fields during the import process.
How to get there
First, create your custom fields.
Second, use the standard import tool to map columns in your spreadsheet to those fields.
Importing into Custom Fields
Custom fields are still data about supporters, so in this case, you'll still import into the default supporter table. Prepare you data for import and save your file following the instructions in the supporter imports documentation.
Create one column per custom field in your spreadsheet.
In step two of the import process, scroll down to the Custom fields section.
If you've created your custom field before the import (recommended), this is very simple. Simply choose the field from the drop down list as you would any other Salsa field.
If you didn't create your custom fields before the import, you can do so now.
Check out the custom field documentation for more information about creating those custom fields.