I saved this post I made a few years back on Gospel Melodies. Someone asked me how do I go about learning a song by ear. This applies to more complicated songs (I don't do this for hymns and easier p&w type songs). But, like that Kurt Carr song or Hour of Visitation Post, and when I learned Jesus I Love recently for someone on here.
Here's my answer to the question asked a few years back....
When I listen to songs I usually:
1. Find the melody (some of it first) and listen for where a song resolves, i.e. at the end of the verse, I can hear that "I" chord (one chord) or the "tonic" chord. That tells me the key the song is in. I have to know the key, then I can figure out the progressions from there, usually. And, I can hear major or minor keys--that just takes time.
2. Next, I'm listening for bass notes. I hear the bottom the best, and the I figure out what chords will go with that bass note. I can also hear "suspended" chords well (a chord w/a sharp 5th) for instance C(#5) is C E G#. I can also hear a bass note with a different chord over it, for instance C\G = LH/RH G\C E G. Listen for your bass notes. Once you have your bass notes and a pattern, go from there.
3. For me, organization is the key. I type out the words, while listening to the CD. I stop, start, stop, start...get all the words typed out with space in between each line to write. This way, I can see and hear, the verses, choruses and vamps. So many songs are so much alike (at least very similar) that once you start to really engage your ears, you find you can play one song, and you can play 4 or 5 others you like.
4. More about the "organization" factor: Successful ear players have a plan, and a method. Find your own, but what I've stated works for me. I need to "see" things. And, while I stop, start, stop, start, typing the words, I'm also hearing the patterns, the chords, and paying attention to what's repetitive in the song.
5. Your melody note: That is the soprano part if it's a choir singing. Build your chord beneath that melody note and you have the choir parts too!
I hope that this answered your questions.
Many Blessings to you!
Lisa