For those who want to go straight to the source, there is the church's official "Manual of Indulgences." It's 172 pages and available online for $19.95. (You also can find copies at Amazon.com, where they're being offered for as little as $12.57.)
Here's a shorthand explanation.
There are two prerequisites for seeking an indulgence: You must be baptized and you must be in "full communion" with the Roman Catholic Church. Beyond that, there are five steps to follow:
• Going to confession.
• Receiving communion.
• Having no attachment to sin.
• Performing an act of love or devotion.
• Saying prayers for the intentions of the Holy Father.