Eureka

Great thanks to Mark John, who brought to my attention the Enchiridion of Indulgences. I had forgotten about this resource, and look forward to exploring it at length. It was issued shortly after the Second Vatican Council by Paul VI, so it's not some backwards anachronism.

"Enchiridion" means "handbook" or "digest", and I found an exceedingly lucid explanation of it at Catholic Online. At the risk of violating copyright, I'll paste the explanation here:

You don't hear about indulgences anymore, at least not in Catholic circles. If it could be said that at one time they were over emphasized, it's surely true that today they're under-emphasized. Many Catholic simply don't know what indulgences are, and they're at a loss to explain the Church's position on indulgences when challenged by fundamentalists.

And fundamentalists do bring up indulgences, perhaps because they know even less about them than the average, poorly-informed Catholic.

There is surely no better place to turn than to the Enchiridion of Indulgences. "Enchiridion" means "handbook," and the Enchiridion of Indulgences is the Church's official handbook on what acts and prayers carry indulgences and what indulgences actually are.

An indulgences is defined as "the remission before God of the temporal punishment due for sins already forgiven as far as their guilt is concerned." The first thing to note is that forgiveness of a sin is separate from punishment for the sin. Through sacramental confession we obtain forgiveness, but we aren't let off the hook as far as punishment goes.

Indulgences are two kinds: partial and plenary. A partial indulgences removes part of the temporal punishment due for sins. A plenary indulgence removes all of it. This punishment may come either in this life, in the form of various sufferings, or in the next life, in purgatory. What we don't get rid of here we suffer there.

snip

Since some Catholics were confused by the designation of days and years attached to partial indulgences, and since nearly all Protestants got a wrong idea of what those numbers meant, the rules for indulgences were modified in 1967, and now "the grant of a partial indulgence is designated only with the words "partial indulgence," without any determination of days or years," according to the Enchiridion.

To receive a partial indulgence, you have to recite the prayer or do the act of charity assigned. You have to be in the state of grace at least by the completion of the prescribed work. The rule says" at the completion" because often part of the prescribed work is going to confession, and you might not be in the state of grace before you do that. The other thing required is having a general intention to gain the indulgence. If you perform the required act but don't want to gain the indulgence, obviously you won't gain it.

The requirements for a plenary indulgence are tougher than for a partial. After all, a plenary indulgence remove all the temporal punishment due for the sins committed up to that time.

(If you sin later, of course, the temporal punishment connected with the new sins isn't covered by the earlier plenary indulgence, but, at least the punishment for the old sins isn't revived.)

"To acquire a plenary indulgence," says the Enchiridion, "it is necessary to perform the work to which the indulgence is attached and to fulfill the following three conditions: sacramental confession, Eucharistic Communion, and prayer for the intention of the Sovereign Pontiff. It is further required that all attachment to sin, even venial sin, be absent."

You can download the Enchiridion as a pdf file after googling it off the Internet.