For High School graduation they trained (in 1 week )the whole flock of us to sing "Sunrise Sunset" in harmony. (Separated the ewes from the rams and taught us to sing one part only of the song). Later they brought us together with the music and each other. Garsh, it sounded so pretty. My mother-in-law can sing harmony as easily as the rest of us tie shoelaces. It is not easy, I have discovered. Singing in the car to well known tunes helps me (as long as the tune is "Sunrise Sunset") because the proximity of the windshield is sort of like the hand-to-mouth/ear technique. You get your own feedback. It takes some concentration to filter out the foreground of a song to hear the other pieces of it. Those of us who had quadraphonic stereos in the olden days were able to "zoom in" on the different layers of the song, and match voices with it. Thanks to Barry and John/Brisbane for the tips on where to get more instruction on harmony singing.
Thanks to all for your harmonious thoughts,