You have to have those separated, the metallic texture should be a separate one from the head, meaning that it will be kind of under the preset, that is why you will need to erase the part you want to be metallic. You also need to code it appropiately to work.
As in the tutorial says ''Now as you may have guessed, we are going to put the metal down first, and your preset's texture goes over it.
So we can do this,''
this explains it.
and the code it's in the thread linked by Dark_Heart.
edit: If you erased his muzzle, the muzzle would be the one showing the metallic texture.
And yes, you will erase the preset itself so the metallic texture, that is under the preset, will show up.