The Garritan samples would need direction ie a midi file. You would write a midi file then have that file 'interpreted' by the Garriatan... whatever sound patch is appropriate.

So in BIAB or Real Band or ? You would apply the Garritan software as a plug-in to your midi track. It does nothing on it's own. If you have a tune with midi strings part written or midi bass etc, Garritan would supply the 'sound' to the notes.