§1. A coadjutor bishop takes possession of his office when he, either personally or through a proxy, has shown the apostolic letter of appointment to the diocesan bishop and college of consultors in the presence of the chancellor of the curia, who records the event.
§2. An auxiliary bishop takes possession of his office when he has shown the apostolic letter of appointment to the diocesan bishop in the presence of the chancellor of the curia, who records the event.
§3. If the diocesan bishop is completely impeded, however, it suffices that both the coadjutor bishop and the auxiliary bishop show the apostolic letter of appointment to the college of consultors in the presence of the chancellor of the curia.
Source: Code of Canon Law (Vatican.va)