A plenary indulgence is "the remission of the entire temporal punishment due to sin so that no further expiation is required in Purgatory" [1
]. It "can always be applied either to oneself or to the souls of the deceased, but they cannot be applied to other persons living on earth" [2
]. Sounds good, no?
There are four plenary indulgences that can be obtained at any time and any place:
- "with the veneration due the divine word [, making] a spiritual reading from Sacred Scripture...for at least one half an hour" 
- recitation of the rosary "in a church or public oratory or in a family group, a religious Community or pious Association"  (see additional norms)
- "make the pious exercise of the Way of the Cross"  (see additional norms)
- "visit the Most Blessed Sacrament to adore it...for at least one half an hour" 
Other things that must be done:
- "Sacramental confession. A single sacramental confession suffices for gaining several plenary indulgences; but Communion must be received and prayer for the intention of the Sovereign Pontiff must be recited for the gaining of each plenary indulgence.
- "Eucharistic Communion.
- "Prayer for the intention of the Sovereign Pontiff. The condition of praying for the intention of the Sovereign Pontiff is fully satisfied by reciting one Our Father and one Hail Mary; nevertheless, each one is free to recite any other prayer according to his piety and devotion.
- "It is further required that all attachment to sin, even venial sin, be absent. If the latter disposition is in any way less than perfect or if the prescribed three conditions are not fulfilled, the indulgence will be partial only, saving the provisions given in Norms 34 and 35."