The following is either condensed or excerpted from “the Rite of Exorcism” as found in the Roman Ritual (the Official Ritual Text of the Catholic Church).
The priest goes to confession and says mass before performing the exorcism (during the exorcism he should be wearing a purple stole). The priest who undertakes the office should be “a holy man, of a blameless life, intelligent, courageous, humble, and he should prepare for the work by special acts of devotion and mortification, particularly by prayer and fasting.”
• The priest traces the sign of the cross over himself, the possessed and all others present and then sprinkles them with holy water.
• The Litany of the Saints is prayed.
• The Our Father is recited and Psalm 53 prayed.
• An invocation is made–“hasten to our call for help and snatch from ruination and from the clutches of the noonday devil this human being made in your image and likeness.”
• The priest then commands the demon to leave “I command you, moreover, to obey me to the letter, I who am a minister of God despite my unworthiness; nor shall you be emboldened to harm in any way this creature of God, or the bystanders, or any of their possessions.”
• The priest then lays hands on the possessed and prays for their healing.
• At least one of five gospel passages is read – Mark 16:15-18
• The priest then begins one of several prayers that order the demons to be cast out of the body of the possessed. All others in the room are encouraged to devoutly pray the Our Father and Hail Mary.
• After the demon is exorcised, the Canticle of Mary and the Canticle of Zachary are prayed and the house is blessed.
Also from the Roman Ritual
• He [the priest] should avoid in a the course of the rite everything that savors of superstition, and should leave the medical aspects of the case to qualified physicians.
• All idle and curious questioning of the demon should be avoided, and the prayers and aspirations should be read with great faith, humility, and fervor, and with a consciousness of power and authority.
• The Blessed Sacrament is not to be brought near the body of the obsessed during exorcism for fear of possible irreverence; but the crucifix, holy water, and, where available, relics of the saints are to be employed.
Example of a prayer of exorcism:
I adjure you, ancient serpent, by the judge of the living and the dead, by your Creator, by the Creator of the whole universe, by Him who has the power to consign you to hell, to depart forthwith in fear, along with your savage minions, from this servant of God, who seeks refuge in the fold of the Church. Yield, therefore, yield not to my own person but to the minister of Christ. For it is the power of Christ that compels you, who brought you low by His cross. Tremble before that mighty arm that broke asunder the dark prison walls and led souls forth to light.