Here are two YouTube lessons that I have done related to singing chords. The more you practice singing the chord tones, the easier it will be to sing a harmony part by ear. In the second YouTube lesson, see if you can add a harmony part while I sing "Down in the Valley". Remember that the easiest way to do this is to sing one chord tone until the chord changes; then move to the closest chord tone in the new chord.
I, IV, V chords introduction
I, V7 chords
Also, remember to listen for the chord changes in any songs you listen to this week. Can you anticipate the chord change? Can you sing the chord tones in your mind or out loud and move to one in the new chord?
Have fun and see you Monday!