On the Sources screen, when you click New (in the Notecards column) to create a notecard, it is automatically associated with the source that you are working with.
If you create a notecard from the Notecards screen, you must select the source from a dropdown list when you are creating the notecard.
When you edit an existing notecard (from either the Sources or Notecards screens), you can change this association by selecting a new source from the Source drop down list on the Edit notecard screen.
To link multiple notecards to a source at once, select any number of notecards on the Notecard Tabletop view (they will display with an orange selection rectangle), then click Link to source above the tabletop. Select the source from the Select source dropdown list and click Submit to save the change.