The exact form of the ritual varies a little based on the mummy's Guild. The Arisen and the would-be Sadikh must exchange something of intrinsic value - sacrificing a loved one, an act of self-mutilation, or their True Names. Then the mummy mystically writes their True Name on the Sadikh's heart, binding them together. The ritual can only be performed on a completely willing subject, though that is not the same thing as a fully informed subject.
In first edition, the Sadikh was always required to give their True Name to their mummy master, and the process of engraving the heart was note described in any detail other than "mystical." The second edition rules require a mutual exchange, though True Names are only one option; it explicitly states that the mummy must cut the Sadikh's chest open and directly inscribe their name on their heart. The wound heals instantly by magic but leaves a distinctive scar.