"An indulgence is the remission of God of a temporal punishment to to sins whose guilt has already been forgiven. The faithful... can acquire this remission through the Church..."Partial indulgences free one from part of the temporal punishment due to sin.
Plenary indulgences free one from the whole of the temporal punishment due to sin.
"In order to gain a plenary indulgence, one must perform the prescribed work and also fulfill three conditions: sacramental Confession, Eucharistic Communion, and prayer for the intention of the Supreme Pontiff."Also, all attachment to sin, even venial sin, must be absent. If any of these prescribed conditions have not been met, then the individual will gain a partial indulgence.
The following are the most common ways of earning a partial indulgence:
1. Acts of faith, hope, charity, and contrition, according to any good formula.
2. A Visit of adoration to the Blessed Sacrament.
3. "We give Thee thanks, Almighty God, for all Thy benefits, Who lives and reigns forever and ever. Amen."
4. Reciting the Prayer to Our Guardian Angel.
5. Recitation of the Angelus or (the Regina Caeli during Easter Time).
6. Recitation of the Anime Christi.
7. For visiting a cemetery and praying for the departed. This is applicable only to the Holy Souls in Purgatory.
8. Spiritual Communion, according to any devout formula.
9. Devout recitation of the Apostles' Creed or the Nicene Creed.
10. For teaching or studying Christian Doctrine.
11. For reciting the Prayer before a Crucifix.
12. Pious recitation of the Act of Reparation.
13. Pious recitation of the Act of Dedication of the Human Race to Christ the King. A Plenary indulgence is gained if recited publicly on the Feast of Christ the King.
14. For each of the following litanies: The Most Holy Name of Jesus, The Sacred Heart of Jesus, The Most Precious Blood of Our Lord Jesus Christ, The Blessed Virgin Mary, St. Joseph and the Litany of the Saints.
15. Recitation of the Magnificat.
16. Recitation of the Memorare.
17. For devoutly making use of an object of piety (sacramentals) properly blessed by any priest.
18. For spending some time in devout mental prayer.
19. "V: Let us pray for our Sovereign Pontiff.N
R: The Lord preserve him and give him life, and make him blessed upon the earth, and deliver him not up to the will of his enemies."
20. For devoutly and attentively being present at a sermon.
21. "Eternal rest grant unto them, O Lord, and let your perpetual light shine upon them. May they rest in peace." This is applicable to the Holy Souls in Purgatory.
22. Recitation of the Holy Rosary privately, outside of a Church or public oratory.
23. For reading Sacred Scripture as spiritual reading, with the veneration and devotion due to the word of God.
24. Recitation of the Salve Regina.
25. For devoutly making the Sign of the Cross, saying the words "In the name of the Father and of the Son and of the Holy Spirit. Amen."
26. Devout recitation of Tantum Ergo.
27. "Come, Holy Spirit, fill the hearts of Thy faithful, and enkindle in them the fire of Thy love."
28. For renewing one's baptismal vows, using any customary formula. A Plenary indulgence is granted if done during the Paschal vigil or on the anniversary of one's baptism.
From Purgatory by Fr. F. X. Schouppe, and published by Tan Books.