A special type of plenary indulgence is available only for Divine Mercy Sunday: Apostolic Penitentiary Decree.
“To ensure that the faithful would observe this day with intense devotion, the Supreme Pontiff himself established that this Sunday be enriched by a plenary indulgence, as will be explained below, so that the faithful might receive in great abundance the gift of the consolation of the Holy Spirit. In this way, they can foster a growing love for God and for their neighbour, and after they have obtained God’s pardon, they in turn might be persuaded to show a prompt pardon to their brothers and sisters.”
The decree goes on to establish the conditions for the plenary indulgence:
“a plenary indulgence, granted under the usual conditions (sacramental confession, Eucharistic communion and prayer for the intentions of Supreme Pontiff) to the faithful who, on the Second Sunday of Easter or Divine Mercy Sunday, in any church or chapel, in a spirit that is completely detached from the affection for a sin, even a venial sin, take part in the prayers and devotions held in honour of Divine Mercy, or who, in the presence of the Blessed Sacrament exposed or reserved in the tabernacle, recite the Our Father and the Creed, adding a devout prayer to the merciful Lord Jesus (e.g. Merciful Jesus, I trust in you!)”
In summary, to obtain complete forgiveness of sin and punishment:
1. make a good Confession
2. receive holy Communion
3. pray for the intentions of Pope Francis
4. pray in front of the Eucharist (exposed or in a tabernacle) at least an Our Father, Creed, and any prayer to the Divine Mercy.
5. detachment from even venial sin
I suggest that it is preferable to receive Communion at a Mass for Divine Mercy Sunday.
I suggest that it is preferable to make your Confession on the Saturday before Divine Mercy Sunday. However, the usual conditions for a plenary indulgence permit reception of the Sacrament of Confession a number of days before or after the work of the indulgence.
I suggest that the prayers include the Our Father, Hail Mary, Creed, and at least one decade of the Divine Mercy Chaplet. As for the prayer for the Pope’s intention, every Catholic should be praying for his intentions at least once a week, if not daily.
However, I dispute whether detachment from even venial sin is necessary for this particular plenary indulgence. The indulgence is based on the revelation of Jesus to Saint Faustina.
Jesus said: “The soul that will go to Confession and receive Holy Communion shall obtain complete forgiveness of sins and punishment. On that day all the divine floodgates through which grace flow are opened. Let no soul fear to draw near to Me, even though its sins be as scarlet.”
It is contrary to these words to claim that complete forgiveness of sin and punishment is only available to souls who are detached from all sin. I would go so far as to say that, even if the person does not happen to pray for the Pope’s intention, but only makes a good Confession, receives Communion worthily, and says any type of prayer for the Mercy of God, the plenary indulgence is obtained.
So in my theological opinion, the Divine Mercy Sunday indulgence is a special type of plenary indulgence, in which the usual conditions of detachment from sin and a prayer for the Pope’s intention are not required. Otherwise, it would not be true that “all the divine floodgates” are opened on that day, and it would not be true that a soul could draw near, without fear, “even though its sins be as scarlet”.
Please take a look at this list of my books and booklets, and see if any topic interests you.