When the priest arrives at the altar, he makes a profound bow. If the Blessed Sacrament is reserved in a tabernacle on the altar, he genuflects (to the floor, not the bottom step of the altar).
Then he ascends the steps, and, placing the chalice on the altar to the left, takes the corporal from the burse. The burse is placed on the Gospel side and the corporal is unfolded in the middle of the altar.
Removing the veil, then the pall, the priest puts the paten with the host on the corporal. The purificator is now removed from the chalice and laid on the altar, along the right side of the corporal.
He takes the chalice in his left hand by the node and goes to the Epistle side. Here he receives in his right hand the cruet of wine from the acolyte and pours a little wine into the chalice to one side.
The priest makes a small Sign of the Cross over the water, saying:
Taking the cruet, he allows a few drops of water to fall into the middle of the chalice.
He returns to the middle of the altar, places the chalice on the corporal and wipes the inside of the chalice with the purificator. He replaces the paten and the host on the chalice, covers it with the pall and the veil and sets the chalice on the corporal in the middle of the altar.
Water ought to be mingled with the wine which is offered in this sacrament. First of all on account of its institution: for it is believed with probability that our Lord instituted this sacrament in wine tempered with water according to the custom of that country: hence it is written (Prov. 9:5): “Drink the wine which I have mixed for you.” Secondly, because it harmonizes with the representation of our Lord’s Passion, hence Pope Alexander I says: “In the Lord’s chalice neither wine only nor water only ought to be offered, but both mixed, because we read that both flowed from His side in the Passion.” Thirdly, because this is adapted for signifying the effect of this sacrament, since as Pope Julius says: “We see that the people are signified by the water, but Christ’s blood by the wine. Therefore when water is mixed with the wine in the chalice, the people are made one with Christ.” Fourthly, because this is appropriate to the fourth effect of this sacrament, which is the entering into everlasting life, hence Ambrose says: “The water flows into the chalice, and springs forth unto everlasting life.”(St. Thomas Aquinas, ST III, q. 74, a. 6)
Going to the missal, he opens it to the Mass to be said and returns to the middle. With both hands he uncovers his head and adjusts the cowl and amice about his neck, saying in a low voice:
Priest:
He turns to the right, goes to the top step (the first step below the predella, that is, the first step below the level where the priest stands while saying Mass) and then faces the altar. If the predella has no steps leading to it, the celebrant stands on the floor.
Water ought to be mingled with the wine which is offered in this sacrament. First of all on account of its institution: for it is believed with probability that our Lord instituted this sacrament in wine tempered with water according to the custom of that country: hence it is written (Prov. 9:5): “Drink the wine which I have mixed for you.” Secondly, because it harmonizes with the representation of our Lord’s Passion, hence Pope Alexander I says: “In the Lord’s chalice neither wine only nor water only ought to be offered, but both mixed, because we read that both flowed from His side in the Passion.” Thirdly, because this is adapted for signifying the effect of this sacrament, since as Pope Julius says: “We see that the people are signified by the water, but Christ’s blood by the wine. Therefore when water is mixed with the wine in the chalice, the people are made one with Christ.” Fourthly, because this is appropriate to the fourth effect of this sacrament, which is the entering into everlasting life, hence Ambrose says: “The water flows into the chalice, and springs forth unto everlasting life.”(St. Thomas Aquinas, ST III, q. 74, a. 6)