Mele,

If you want to try brazing, you might want to try some regular mild steel first. Cast iron is about the toughest ferrous metal there is to braze successfully. Because of the different cross sections and stresses created by the casting itself, the stuff tends to expand with heat in ways the make it try to tear itself apart. Often, you get more cracking during the brazing process. That said, the general concept for cast iron is this:

First, clean it thoroughly with a wire wheel or brush, removing all rust from the braze area and immediately around it. Next, if the piece has differing thicknesses, you need to pre-heat the whole thing to about 400 degrees.

After the preheat, you can use the torch to heat the place you want to braze. If you're using plain brazing rod, you'll want to heat a couple of inches of the rod to a red heat and dip in some brazing flux. The flux will adhere to the rod and transfer to the workpiece when the work is hot enough. So, you heat the joint, bring the fluxed rod into the flame, and let the brass flow into the joint.

After you get the joint brazed, set the piece aside to cool slowly - the slower the better. If it is small enough, I recommend burying it in ashes or vermiculite to insulate it. If you try to cool it quickly, it will set up stresses and crack in another place.

I hope this makes sense.