As Rose and others said, there are two things you can do for Divine Mercy Sunday.
The special graces from Jesus are not an “indulgence”, technically. Websites and other people keep calling them an “indulgence”. Actually, an “indulgence” is a way to reduce temporal punishment that is provided by the Church under its binding and loosing authority. So if grace and mercy is coming straight from Jesus, it’s not coming from the Church and is not under the church authority, and is not an “indulgence”.
Here is more on the difference:
(1) To get the special graces from Jesus:
- go to confession during a reasonable time prior to Divine Mercy Sunday (I understand that even going during Holy Week is sufficient, provided you stay in the state of grace)
- be in a state of grace
- receive Holy Communion on Sunday, or at the Saturday vigil Mass for Sunday, with trust in Jesus’ Divine Mercy.
Note that this special grace is just for you. You can’t give it away to a poor soul in purgatory. You can of course pray for the poor soul in Purgatory or offer your Mass/ Communion for them but that’s not the same as giving them the grace from Jesus.
(2) To get a Plenary Indulgence (provided by the Church, not by Jesus) for Divine Mercy Sunday, you do the things Jubilant said.
You can use the same confession (provided it’s within 20 days and you stay in the state of grace) as in (1).
You can also use the same Holy Communion as in (1).
Note however that the work (participating in the Divine Mercy service or saying the prayers as explained by Jubilant) needs to be done ON THE SUNDAY, not at the Saturday vigil.
Some of the churches in my area have special Sunday prayer services for Divine Mercy, so people who went to Saturday Mass can come to the Sunday prayer service in the afternoon for their Divine Mercy devotions. Maybe you can find a prayer service in your area.
This indulgence can be given to a soul in purgatory - and frankly, if you’re receiving Jesus’ special graces already, it makes sense to give the indulgence to a soul because you’re not going to need it having gotten the grace from Jesus.
I hope this helps