Guidelines to receive Holy Communion
The minimum fast before receiving Holy Communion is one hour before the reception of Holy Communion, not the start of Mass. This fast includes all food and drink except for water and medicine.
How to Receive Communion:
General Instruction of the Roman Missal, paragraph 160:
... the communicants, ... 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.... 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.
Based on clarifications from the Vatican after the General Instruction was issued, it is clear that this paragraph is to be understood as requiring those receiving Holy Communion to approach in a procession and at the very least bow their heads before receiving Communion standing. The Vatican has been clear that those receiving Communion may genuflect or kneel, but the minimum sign of reverence is a bow of the head before receiving Communion.
Communion on the tongue: Receiving Communion on the tongue is a centuries-old practice that is still encouraged by the Vatican. When you approach the minister for Holy Communion, respond "Amen" after the words "The Body of Christ." Then, open your mouth with your tongue relaxed against the bottom teeth and lips.
Communion in the hand: There is only one correct way to receive Communion in the hand. Place your dominant (or writing) hand underneath your other hand, with the palms facing up. Once the host has been placed in the palm of the hand on top, move your dominant hand out from underneath, take the host, and place it in your mouth.
Guidelines for Receiving Holy Communion from the Catechism of the Catholic Church:
1384 The Lord addresses an invitation to us, urging us to receive him in the sacrament of the Eucharist: "Truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you."
1385 To respond to this invitation we must prepare ourselves for so great and so holy a moment. St. Paul urges us to examine our conscience: "Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For any one who eats and drinks without discerning the body eats and drinks judgment upon himself." Anyone conscious of a grave sin must receive the sacrament of Reconciliation before coming to Communion.
1388 It is in keeping with the very meaning of the Eucharist that the faithful, if they have the required dispositions, receive Communion when they participate in the Mass. As the Second Vatican Council says: "That more perfect form of participation in the Mass whereby the faithful, after the priest's Communion, receive the Lord's Body from the same sacrifice, is warmly recommended."
1389 The Church obliges the faithful to take part in the Divine Liturgy on Sundays and feast days and, prepared by the sacrament of Reconciliation, to receive the Eucharist at least once a year, if possible during the Easter season. But the Church strongly encourages the faithful to receive the Holy Eucharist on Sundays and feast days, or more often still, even daily.
1398 The Eucharist and the unity of Christians. Before the greatness of this mystery St. Augustine exclaims, "O sacrament of devotion! O sign of unity! O bond of charity!" The more painful the experience of the divisions in the Church which break the common participation in the table of the Lord, the more urgent are our prayers to the Lord that the time of complete unity among all who believe in him may return.
1399 The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. "These Churches, although separated from us, yet possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy." A certain communion in sacris, and so in the Eucharist, "given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged."
1400 Ecclesial communities derived from the Reformation and separated from the Catholic Church, "have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders." It is for this reason that, for the Catholic Church, Eucharistic intercommunion with these communities is not possible. However these ecclesial communities, "when they commemorate the Lord's death and resurrection in the Holy Supper . . . profess that it signifies life in communion with Christ and await his coming in glory."