It really doesn't matter. In my experience, a song can be written using a wild assortment of procedures.
A few examples:
- Write the lyrics for the verse, write the music for that, then write the music for the chorus/hook... and get the inspiration from that for the chorus lyrics. Then build from that point.
- Write the chorus first, complete with lyrics. When you're satisfied, add the part(s) that will lead to that.
- Think of a subject/theme for the song first. Then try to imagine what kind of musical atmosphere would better convey it. With this in mind, start writing bits and pieces of lyrics and music, and go on from there like a puzzle.
- On the contrary, imagine a musical mood - just think of it for a while. Then try to put it into practice by writing the music for a whole part of the song - typically a verse. From there, you can proceed either by writing lyrics for that, or completing the musical part and think about lyrics later.
- No need to be obvious: sometimes, it's very effective to put strong lyrics on a delicate musical bed, for example.
- and - of course! - always write the intro and the coda for last.