Be in a state of grace when the indulgenced work is completed;
Have the interior disposition of complete detachment from sin, even venial sin;
Have sacramentally confessed one’s sins;
Receive the Holy Eucharist;
Pray for the intentions of the Pope.
The Church also adds some extra clarification to some of these conditions:
A plenary indulgence can only be acquired once per day (ID, norm 6).
One may make a sacramental Confession and receive Holy Communion “within several days before or after the prescribed work” (ID, norm 8).
One sacramental Confession can satisfy the requirement for several plenary indulgences, but separate receptions of Holy Communion and prayers for the Holy Father are required for each plenary indulgence (ID, norm 9).
An Our Father and a Hail Mary are suggested prayers for the Pope’s intentions (ID, norm 10).
Indulgences can either be applied to oneself or to the souls of the deceased, but not to other living persons (ID, norm 3). It is worth noting that some indulgences can only be applied to the souls of the faithful departed, like the ones offered by the Church in early November for the Commemoration of the Faithful Departed (All Souls).