The Divine Mercy Chaplet was received from Jesus by St Faustina in the early 20th century.
St Faustina was instructed: Encourage souls to say the Chaplet which I have given you (1541). Whoever will recite it will receive great mercy at the hour of death (687). When they say this Chaplet in the presence of the dying, I will stand between My Father and the dying person, not as the just Judge but as the Merciful Saviour (1541). Priests will recommend it to sinners as their last hope of salvation. Even if there were a sinner most hardened, if he were to recite this Chaplet only once, he would receive grace from My infinite mercy (687). I desire to grant unimaginable graces to those souls who trust in My mercy (687). Through the Chaplet you will obtain everything, if what you ask for is compatible with My will. (1731) (The numbers are the section references in the diary of St Faustina) There are many online resources, particularly the extensive https://www.thedivinemercy.org/ The chaplet can be said at anytime, but with a specific request from Jesus that it should be said as a novena before the Divine Mercy Sunday (second Sunday of Easter). The chaplet is commonly said at 3 o’clock in the afternoon, the hour of mercy when Jesus died. It commences with the Divine Mercy 3 o’clock Prayer Then using a rosary to regulate the rhythm of the prayers.