"An indulgence is a remission before God of the temporal punishment due to sins whos guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, despenses and applies with authority the treasury of the satisfactions of Christ and the saints."
 - Catechism of the Catholic Church, 1471

Decree on Indulgences for the Year of St. Joseph: English | Spanish (Español)

Excerpts from the Decree on Indulgences in the Year of St. Joseph of the means to gain plenary indulgences

The Plenary Indulgence is granted under the customary conditions (sacramental confession, Eucharistic communion and prayers according to the Holy Father's intentions) to the faithful who, with a soul detached from any sin, shall participate in the Year of Saint Joseph, in the occasions and with the modalities indicated by this Apostolic Penitentiary. 

1. Contemplate the Lord's Prayer for at least 30 minutes, or participate in a Spiritual Retreat of at least one day which involves a meditation on St. Joseph

2. Fulfill a corporal or spiritual work of mercy, after the example of St. Joseph

3. Recite the rosary as a family and among betrothed (engaged couples)

4. Entrust their daily life to the protection of St. Joseph. All faithful who shall invoke through prayer the intercession of the Worker of Nazareth, so that those in search of work may find employment and the work of all people may be more dignified

5. Recite the Litanies to Saint Joseph (Latin) or the Akathistos to Saint Joseph, whole or in part (Byzantine) or some other prayer to Saint Joseph for the Church persecuted and for the relief of all Christians who suffer any form of persecution

In addition to the above, a plenary indulgence is granted to any faithful who shall recite an approved prayer or do an act of piety in honor of St. Joseph, especially on the occassions of: March 19th, May 1st, Feast of the Holy Family, Sunday of St. Joseph (Byzantine), the 19th of every month, and every Wednesday. 

In the current context of the health emergency, a plenary indulcence extends particularly to the elderly, the sick, the suffering, and all those who for legitimate reasons are unable to leave the house, and who with a soul detached from any sin and the intention to fulfill the three customary conditions in their own place shall recite an act of piety in honor of St. Joseph, Comfort of the Sick and Patron of a Happy Death, offering to God their suffering and the hardships of their life. 

15 ways to gain an indulgence in the Year of St. Joseph - Catholic News Agency