Answer:
Melodies and lyrics are the two components of any song that must be unique. Song titles, chord progressions, rhythms… they can all have been heard before, and probably have. But melodies, if they share more than a few notes in a row with some other song, are said to have been plagiarized.
Because every melody needs to be completely different from every other melody, it’s difficult to come up with a process that says “Here’s how you write a good melody.” It’s possible to come up with a step-by-step process (I’ve done that), but when all is said and done, a process will only get you so far.
