Ring Doorbell 2: How can we make the wired installation

In a few words, I wanna know how can install my Ring Doorbell 2 using the mechanical bell installed in the house. I'm clear the diode shall be used only if we have a digital bell. It is not clear to me if we have a mechanical bell with a 8 -24 Vac, why we need to use a transformer? Also Ring videos say something about a resistor. Why, where and when we need to use this resistor?