According to the current Enchiridion of Indulgences, one can apply a plenary indulgence to a departed soul by the “visitation of a cemetary” {Coemeterii visitatio} from November 1st till the 8th (i.e. the octave of All Saints).

Here’s the text:

13. Visit to a Cemetery (Coemeterii visitatio)

An indulgence, applicable only to the Souls in Purgatory, is granted to the faithful, who devoutly visit a cemetery and pray, even if only mentally, for the departed.

The indulgence is plenary each day from the 1st to the 8th of November; on other days of the year it is partial.

In order for the indulgence to be plenary, the following conditions must also be met alone with the visit and prayers at the cemetery:

1. Sacramental confession within “about twenty days”[1] of the actual day of the Plenary Indulgence.

2. Eucharistic Communion on the day of the Plenary Indulgence.

3. Prayer for the intentions of the Pope on the day of the Plenary Indulgence.

4. It is further required that all attachment to sin, even venial sin, be absent.[2]