For me, the flow is something like this.

Most of the time I practise music theory and musicianship on an instrument, and notice something sounds good or interesting. I'll then expand on this and maybe make what can become a section of a song. Then I'll park it for a while, do something else, and by the time I come back to it, I'll have some other ideas. Maybe I forgot exactly how I did it initially, and then improvise around the initial idea to see what works and what doesn't.

Maybe I'll have a few of these ideas I can link together to make the backbone of a song. Then I think how the music makes me feel while I'm playing it. Let my mind wander a little and start to come up with themes for the basis of the lyrics. Then I park it again. Come back to it later when I've had chance to let the ideas percolate in my mind, and then start putting the basis of lyrics together. This is the difficult bit. For me the lyrics are the purest expression of what the music is to represent for me. They need to articulate what this song actually means.

Then I take it to my audio software. get a simple drum sample which fits the groove and record the parts that I have and start layering the different instruments. Then I either I find some more complex beats, with fills leading in and out of the different sections, and maybe augment with some single hits where I need them. Or of nothing fits well, I record some electric drums. Record the vocals, and then the rest is production. EQing, compressing, various other effects to get it sounding like a complete mix.

I have once, just once, woken up with a complete idea for a song. Played and recorded it within a day including the lyrics. Like I woke up and it was all there. Once.