From Fr. John Hardon's Modern Catholic Dictionary:
PLENARY INDULGENCE
An indulgence that can remove all the temporal punishment due to forgiven sin. No one but God knows for certain when a plenary indulgence is actually gained, because only He knows whether a person's dispositions are adequate. One norm for such dispositions is that "all attachment to sin, even venial sin, be absent." If these dispositions are in any way less than complete, the indulgence will only be partial. The same provision applies to the three external conditions necessary to gain a plenary indulgence: sacramental confession, Eucharistic Communion, and prayer for the intentions of the Pope. If these conditions are not satisfied, an otherwise plenary indulgence becomes only partial. These conditions may be satisfied several days before or after the performance of the prescribed work, though preferably Communion should be received and the prayers offered for the Pope on the same day as the indulgenced work. A plenary indulgence can be gained only once a day.
From the Priestly Fraternity of Saint Peter’s Holy Cross Chaplaincy:
Six General Rules for Obtaining a Plenary Indulgence
State of grace…when performing the indulgenced act
Complete detachment from sin, even venial sin
Confession (20 days before or after the indulgenced act)
Communion (20 days before or after the indulgenced act)
Prayers for the Supreme Pontiff (20 days before or after the indulgenced act)
Indulgenced Act: a special good work with…conditions of place and time
Indulgenced Acts to be Performed for Obtaining a Plenary Indulgence for the Poor Souls of Purgatory
From November 1 to 8: visit of a cemetery with mental prayer for the poor souls
On November 2: visit of a church or an oratory with one Our Father and one Creed being recited