83.104Apply it to “Happy Birthday”

Use the public-domain melody exactly first.

Then on a second pass:

Phrase 1: original harmony.

Phrase 2: design a smoother or more chromatic bass.

Phrase 3: use the climax melody note as an anchor while inner harmony moves.

Phrase 4: create a bass descent into final tonic.

The reharmonization should sound like the familiar melody has acquired a richer undercurrent—not like the tune has been replaced by a chord exercise.