The spell itself required an object that held the soul of the deceased and a vessel who carried the bloodline of the deceased.
The spell also required use of advanced technology in order to cast it, in addition to the use of magical acts like mixing/brewing odd ingredients, spellwork, and spell recitation. As shown in "The One You've Been Waiting For", the object used to hold Hitler's soul, the Nazi Pocket Watch, was specifically engineered to contain it and make the spell-casting automated.If the ritual was to involve a vessel other than that of the original owner and unrelated by blood, a large transfusion from a blood relative of the soul's original owner was required. In order to accomplish this, an advanced blood transfusion machine connecting the vessel with the blood relative was needed.
After the vessel's body was prepared, the caster placed the object that contained the soul atop their chest. After that, the container activated and the mechanism carved a swastika-shaped incision into the vessel's flesh and entered the body, integrating with the heart. Another mechanism of the object sealed the wound. After the procedure was completed, the transplanted soul was able to take control of the vessel's body.