155. After this, the Priest takes the host, breaks it over the paten, and places a small piece in the chalice, saying quietly, Haec commixtio (May this mingling). Meanwhile the Agnus Dei (Lamb of God) is sung or said by the choir and by the people (cf. no. 83).
156. Then the Priest, with hands joined, says quietly the prayer for Communion, either Domine Iesu Christe, Fili Dei vivi (Lord Jesus Christ, Son of the living God) or Perceptio Corporis et Sanguinis tui (May the receiving of your Body and Blood).
157. When the prayer is concluded, the Priest genuflects, takes a host consecrated at the same Mass, and, holding it slightly raised above the paten or above the chalice, facing the people, says, Ecce Agnus Dei (Behold the Lamb of God) and together with the people he adds, Lord, I am not worthy.
158. After this, standing facing the altar, the Priest says quietly, Corpus Christi custodiat me in vitam aeternam (May the Body of Christ keep me safe for eternal life), and reverently consumes the Body of Christ. Then he takes the chalice, saying quietly, Sanguis Christi custodiat me in vitam aeternam (May the Blood of Christ keep me safe for eternal life), and reverently partakes of the Blood of Christ.
159. While the Priest is receiving the Sacrament, the Communion Chant begins (cf. no. 86).
160. The priest then takes the paten or ciborium and goes to the communicants, who, as a rule, approach in a procession.
The faithful are not permitted to take the consecrated bread or the sacred chalice by themselves and, still less, to hand them from one to another. The norm for reception of Holy Communion in the dioceses of the United States is standing. Communicants should not be denied Holy Communion because they kneel. Rather, such instances should be addressed pastorally, by providing the faithful with proper catechesis on the reasons for this norm.
When receiving Holy Communion, the communicant bows his or her head before the Sacrament as a gesture of reverence and receives the Body of the Lord from the minister. The consecrated host may be received either on the tongue or in the hand, at the discretion of each communicant. When Holy Communion is received under both kinds, the sign of reverence is also made before receiving the Precious Blood.
161. If Communion is given only under the species of bread, the priest raises the host slightly and shows it to each, saying, Corpus Christi (The Body of Christ). The communicant replies, Amen, and receives the Sacrament either on the tongue or, where this is allowed and if the communicant so chooses, in the hand. As soon as the communicant receives the host, he or she consumes it entirely.
If, however, Communion is given under both kinds, the rite prescribed in nos. 284-287 is followed.
162. The priest may be assisted in the distribution of Communion by other priests who happen to be present. If such priests are not present and there is a very large number of communicants, the priest may call upon extraordinary ministers to assist him, i.e., duly instituted acolytes or even other faithful who have been deputed for this purpose.[97] In case of necessity, the priest may depute suitable faithful for this single occasion.[98]
These ministers should not approach the altar before the priest has received Communion, and they are always to receive from the hands of the priest celebrant the vessel containing either species of the Most Holy Eucharist for distribution to the faithful.
From: GENERAL INSTRUCTION OF THE ROMAN MISSAL