Simultaneously. A phrase may start the process--either words or melody; maybe a riff or chord progression; a hook--and then things take shape or not.

On occasion I have heard a melody in my head and had to get someplace real fast to use a guitar or keyboard. That cements it. Occasionally, I have had a stanza done before getting to the harmonic instrument (will a relative minor sound better than a major?). Neat question Jerry. Wish I had a better answer than this.