How do you start writing a lyric if you have the melody and chords finished? You're home all alone sitting at the piano playing the song numerous times and trying to hear and feel how the song, the emotional quality of sound, speaks to you. How does the song mean? You're not writing a musical where you have a story to follow. You don't have a particular scene in which the song would fit. Where do you start? Are you thinking in terms of what the market demands or are you trying to write to express your personal feelings. All these kinds of decisions swirl through your head while you sit there frozen in deciding.
So, perhaps, you create a scene in your mind. A simple scene in which the song, the story, the emotion can find context.
I start by what James Joyce called, "The Stream Of Consciousness!" I corral what is called the "editor" and put "it" in its proper place.