The forms of devotion to Divine Mercy transmitted through Sister Faustina are:
Veneration of the image of Merciful Jesus;
the Feast of Mercy;
the Chaplet to Divine Mercy;
the Hour of Mercy;
the diffusion of the cult of Divine Mercy;
The Apostleship of Divine Mercy.

Divine Mercy

The fame of his holiness grew together with the spread of the cult of Divine Mercy and the graces obtained through his intercession. On February 22, 1931, Sister Faustina wrote in her diary: «In the evening, while I was in my cell, I saw the Lord Jesus dressed in a white robe: one hand raised to bless, while the other touched the robe on my chest, which there it let out two large rays, one red and the other pale. [...] Jesus said to me: "Paint an image according to the model you see, with the inscription below: Jesus, I trust in You! I want this image to be venerated [...] in the whole world. I promise that the soul who will venerate this image will not perish. [...] I want the image [...] to be solemnly blessed on the first Sunday after Easter: this Sunday must be the Feast of Mercy." [7]
Pope Saint John Paul II wrote an encyclical: Dives in Misericordia, the second of his pontificate (1980), entirely dedicated to the devotion learned from the humble Polish nun and it was he who proclaimed her a saint on April 30, 2000. In on that occasion the Pope established for the first time the Feast of Mercy, to be celebrated every year on the first Sunday after Easter.

Chaplet of Divine Mercy

The same topic in detail: Chaplet to Divine Mercy.
In a private revelation in 1935, Jesus allegedly asked Sister Faustina for a particular form of prayer known as the "Chaplet of Divine Mercy". God's mercy, the grace of conversion and the forgiveness of sins, especially at the hour of death, would have been granted to the soul who had recited the chaplet of divine mercy[8]: «My mercy will envelop in life and especially in the hour of death the souls who will recite this chaplet»[9].