From the Pieta prayer book:
Pray for the Souls in Purgatory
The Holy Souls in purgatory are incapable of helping themselves. They depend on the works of mercy by the living and the mercy of Jesus and Mary to shorten their time in purgatory. Their souls are very dear to God's Heart. He is most generous to those who offer every good act, suffering, prayers, or Holy Mass for these souls. In gratitude they intercede for us.
(St. John Vianney once said: "The good God will render us back the good we do for them - poor souls - a hundredfold."
On August 16, 1969 in San Damiano, Italy, Our Lady asked that we pray very much for the poor souls and that if we pray these prayers, we'll "deliver so many souls, so many souls!" Note: The Pieta prayer book may have been referring to this.
"Pray: 5 Apostle's Cred; 1 Hail Holy Queen; 1 Our Father; 1 Hail Mary; 1 Glory be; 1 Requiem: Eternal rest grant unto them, O Lord, and let perpetual light shine upon them, and may they rest in peace. Amen."
Requiem Indulgenced by Pope Leo XIII, March 22, 1902.