There are two things to be aware of: 1) The Church offers a plenary indulgence on this day. 2) Jesus promised special graces on this day. The indulgence and the promise are similar, but not identical.
1) The plenary indulgence: This requires confession (up to 20 days before or after), communion (a couple days before or after), prayers for the Pope (Our Father, Hail Mary), no attachment to any sin, and the specific "work" is the mercy devotions. The devotions are those held in common in the church, or before the tabernacle to pray an Our Father and the Creed and "Jesus, I trust in you". Those who are unable to travel to church can say the Our Father, Creed, and invocation of mercy even at home before an image of the Divine Mercy (but they still must confess and receive communion).
2) The promise of Jesus: That any who confess (up to two weeks before) and receive communion on Divine Mercy Sunday itself (Our Lord does not say that the Saturday evening Mass counts) trusting in his mercy, will have not only their sins but even all the punishment of purgatory will be completely washed away. This is the perfect renewal of the graces of baptism -- and it does not require complete detachment from all sin, but only that we trust in his mercy!
A comparison between Jesus' appearance to Thomas the Apostle and his gift of Divine Mercy Sunday.
Listen online [here]!