In most cases, I am playing around with chord
progression and or riffs on various instruments (mostly guitar or keys - sometimes on drums or bass or other instruments like mandolin, etc.) - I then start to "scat" sing which eventually reveals a few lines that become part of a verse or chorus - from there I continue to work until I have a song form. Once I have what I beleive is a solid song form, I will then start to work on lyrics. Often, I may "re-write" several times as I strengthen the melody, etc.
Once I have a "song" I start mentally forming an arrangement (what instruments do I think are needed for the song, what type of drum groove, what will the bass line be, etc.). This is often done outside of
my studio. So by the time I step into
the studio and hit the record button, I already know what instruments I plan to record in what order, etc.
Now, once I start recording I often decide the bass line I had heard in my head is not working well with the drum groove, etc - so I often change parts of the arrangement while recording individual parts.
I think there were only two occasions when an entire song came to me, lyrics - melody - general rhythm, etc. without an instrument in my hand (in both cases I was driving out of town and had to pull over to write down the lyrics, etc. before the muse stole them back). I then figured out the key and developed an arrangement when I returned home.
I now always have a small digital recorder with me (about the size of a cell phone) to allow me to hum a melody or dictate some lyrical ideas while driving, etc.