Every Mass that we celebrate is directed by the Bishop whether the Bishop is there in person or whether he has appointed a priest to celebrate the Mass (General Instruction for the Roman Missal #92). When the Bishop is present at the Mass, he will often preside over the entire Liturgy. In the event he does not preside for the entire Mass, he will at least preside over the Liturgy of the Word and give the Final Blessing. There are special procedures regarding what to do when a Bishop is present at Mass. The most visible procedures will be regarding the bishop’s mitre and crozier (two of the Bishop’s liturgical regalia). Throughout the Mass, there are times when the Bishop will need to wear his mitre and hold his crozier and then other times when the altar server will need to hold these items for the Bishop. You can find the detailed procedures for Mass with a Bishop, in the Ceremonial of Bishops available through Liturgical Press. If you want more information without buying the book from Liturgical Press, schedule a meeting with your pastor or someone who has been a Master of Ceremonies at a Mass with a Bishop present. These people can walk you through the procedures specifically.