I don’t know where to start. The whole thing is just mysterious to me.
Firestreamer is used in almost exactly the same way as a physical tape library. The only difference is that the tapes are virtual. Familiarize yourself with Firestreamer Basic Concepts and the documentation for your backup software, specifically, with its chapter on tape libraries. If you use Microsoft DPM, we strongly recommend that you watch the demo video before you perform your first backup and restore.