People become possessed by wicked, evil demonic spirits when a relationship is formed between spirit and human. The relationship can be hard to break and can require much prayer to sever the bond. If you are concerned that a friend or loved one might be possessed by an evil or demonic spirit, an exorcism should be performed. Removing an evil spirit from a person by performing an exorcism can be a challenge and should be supervised by a priest.
Pray for deliverance from evil. Many prayers are specific to evil spirits. These prayers are simple exorcisms performed on a daily basis on your own. For instance, Prayer Against Every Evil reads, "Spirit of our God, Father, Son, and Holy Spirit, Most Holy Trinity, descend upon me. Please purify me, mould me, fill me with yourself, and use me. Banish all the forces of evil from me, destroy them, vanquish them so that I can be healthy and do good deeds.
Banish from me all spells, witchcraft, black magic, malefice, ties, maledictions, and the evil eye; diabolic infestations, oppressions, possessions; all that is evil and sinful; jealousy, perfidy, envy; and physical, psychological, moral, spiritual, and diabolical ailments.
I command and bid all the powers who molest me --- by the power of God Almighty, in the name of Jesus Christ our Savior --- to leave me forever, and to be consigned into the everlasting lake of fire, that they may never again touch me or any other creature in the entire world." There are many other prayers that may be specific to your situation and recited for simple removal of evil spirits. (See Reference 3)
Confess your sins and seek the advice of your priest. Some situations may only need prayer for delivery and not actual exorcism. Confessing your sins and recognising the presence of an evil spirit is the first step in abolishing the evil. Even if you are not Catholic or of a certain religious denomination, don't hesitate to ask a priest or pastor for some guidance, as you may not actually be possessed.
Locate an exorcist priest. A parish priest can provide you with information on appointed exorcists in the diocese. Alternatively, call the chancery of the diocese of your church or episcopal curia. The diocese keeps an up to date record on approved exorcists and exorcisms. Also, know that the formal exorcism requires episcopal permission.
Contact the exorcist priest of your diocese and discuss your situation. Schedule a meeting with the priest to pray and discuss the matter at hand.