When you're happy with the settings, just click the “Update widget” button; you'll see the changes applied to the widget in the “Output example” display.
That's it - the widget is ready to use. Just copy and paste the code from the “Code to implement” box to the place in your web page source code where you want the widget to be displayed.
The purpose of custom item lists is for you to be able to display specific item lists to your patrons. Examples for lists are “Latest collection additions”, “Librarian picks”, etc. Follow these steps to create a custom list: