From the Manual on Indulgences, published 2006 by the USCCB, being a translation of the Enchiridion Indulgentiarum quarto editur.
7 EUCHARISTIC ADORATION AND PROCESSION
§1 A plenary indulgence is granted to the faithful who
1. Visit the Blessed Sacrament for adoration lasting at least a half hour;
2. piously recite the verses of the Tantum ergo after the Mass of the Lord’s Supper on Holy Thursday during the solenm reposition of the Most Blessed Sacrament;
3. devoutly participate in a solemn Eucharistic procession, held inside or outside a church, of greatest importance on the Solemnity of the Body and Blood of Christ;
4. participate religiously in the solemn Eucharistic cele¬bration which is customarily held at the conclusion of a Eucharistic congress.
§2 A partial indulgence is granted to the faithful who
1. visit the Blessed Sacrament for adoration;
2. offer any duly approved prayer to Jesus present in the Blessed Sacrament (e.g., the Adoro te devote, the prayer 0 sacrum convivium, or the Tantum ergo).
O Sacrum Convivium
O sacred banquet, in which Christ is received, the memory of his Passion is renewed, the mind is filled with grace, and a pledge of future glory is given to us.
Down in adoration falling,
Lo! the sacred Host we hail;
Lo! o’er ancient forms departing,
Newer rites of grace prevail;
Faith for all defects supplying,
Where the feeble senses fail.
To the everlasting Father,
And the Son who reigns on high,
With the Holy Spirit proceeding
Forth from each eternally,
Be salvation, honor, blessing,
Might and endless majesty Amen.
V. You have given them bread from heaven,
R. Having all delight within it.
Let us pray
O God, who in this wonderful Sacrament left us a memorial of your Passion: grant, we implore you, that we may so venerate the sacred mysteries of your Body and Blood, as always to be conscious of the fruit of your Redemption, You who live and reign forever and ever. Amen.
(Holy Communion and Worship of the Eucharist Outside of Mass; June 21, 1973, nos. 200, 192)