Until recently, indulgences were “measured” in terms of their equivalence to an amount of time of public penance. So if you saw something that had 100 days’ indulgence, it was equivalent to doing 100 days of public penance. A common misconception was that it would take 100 days off your time in Purgatory. Since we don’t know even if there IS time in Purgatory, we know that can’t be correct.
Now, these indulgences are simply called “partial,” since they really can’t be measured.
The other type is the “plenary” indulgence, which remits all of the temporal punishment due to sin. If you gained a plenary indulgence and died immediately after, you would go straight to heaven. In order to gain plenary indulgence, a person must go to confession within two weeks of performing the indulgenced work (prayer, pilgrimage, etc.), go to Mass and receive Holy Communion on the day one does the work, pray for the intentions of the Pope, and be totally detached from sin, even venial sin. If you miss any of the requirements, you will still receive a partial indulgence.
Almost any prayer you say will have some kind of indulgence attached to it. It’s a good thing to form the habit of intending to receive all the indulgences you can each day. You might mention this as part of your morning prayers.
There are a number of things to do to gain a plenary indulgence, some of which are: the Stations of the Cross, spending a half hour reading the Scriptures, saying the Rosary with others in a church. I’ll leave it to the next poster to give you a better list.
Hope this helps!