I learned to harmonize by having an older brother and sister who'd sing with me....became especially good at doing something like an alto part before the voice changed. Later, I'd sing with Simon and Garfunkle recordings and fill in what would be the third harmony part. A lot of it can be done by ear, but depends a lot on what your ear has been acustomed to hearing,.I heard a good deal of PP&M as a kid, so that sort of sound was in my ear all along. Knowing theory is good, but when it starts bogging you down, go for the ear. By the way, in some eastern cultures, the concept of harmony is totally foreign to the music. What happens vertically (ie, harmonically) is purely coincidental.