§1. Where there is a lack of priests and deacons, the diocesan bishop can delegate lay persons to assist at marriages, with the previous favorable vote of the conference of bishops and after he has obtained the permission of the Holy See.
§2. A suitable lay person is to be selected, who is capable of giving instruction to those preparing to be married and able to perform the matrimonial liturgy properly.
Source: Code of Canon Law (Vatican.va)