The Forged are people that are ripped their emotions and connections to all things living. Their soul and humanity are gone. They think only of themselves and their pleasure or needs at the current moment. They can't plan ahead or really work together.
The technique of actual forging isn't known. It was used during the Red Ship Wars by Kebal Rawbread and it was somehow involved with memory stone. The Red Ship Raiders took prisoners and demanded ransom for not releasing them. This was first laughed at and no ransoms were paid. The prisoners that were returned were forged and caused a lot of sorrow to their relatives, as there was no cure to a forged one but death. When this became known, then if Raiders came, people rather took their own lives and those of their children than allowed themselves to be taken prisoners and forged.