Sing In Parts initially comes with Amazing Grace. To unlock a larger selection of hymns, click the sync icon in the upper right corner.
Then choose whether you want to see the entire list of songs or just the hymns for this week.
From the list of songs, select the hymn you would like to learn and practice.
Adjust the volume of each voice.
If you are learning a part for the first time, turn all the other voices down and your voice up. Then press Play. Once you have mastered your part, turn up the other voices and turn down your part to practice singing your part when it is harder to hear it.
Make learning even easier by slowing down the tempo. Gradually increase the tempo until you are up to speed.