This devotion was revealed to St. Faustina (a Polish sister who died in 1938). It revolves around trust in the Lord and appealing to Him for mercy. It is a very powerful prayer and has many wonderful promises attached to it. Our Lord said to St. Faustina:
To pray the Chaplet, you use a Rosary beads. Begin with: One "Our Father"...., One "Hail Mary", and the "Apostles Creed".
Prayers of the Rosary
for these prayers).
Then, on the "Our Father" beads, pray the following:
"Eternal Father, I offer you the Body and Blood, Soul and Divinity of Your dearly beloved Son, Our Lord Jesus Christ, in atonement for our sins and those of the whole world."
On the "Hail Mary" beads, pray the following:
"For the sake of His sorrowful Passion, have mercy on us and on the whole world."
In conclusion, say:
"Holy God, Holy Mighty One, Holy Immortal One, have mercy on us and on the whole world"