Seems this is a slow time of year for homerecording.com and other sites like this. The summer really rocked here.
As to your post, I think it's ok to get one of these every now and then, and it's not by any means spammish.
So for the purposes of this thread I will use the song I currently have up as the model and give a general comment also.
When I hear Scott Kern's music, (Skern), which was designed to have lyrics written into it, I instantly realized it had good structure for a song and saw that structure.
Just like a short story or even more so, like a play, there are acts, scenes, soliloquy, and 'chorus' parts. So seeing the structure and breathing life, (a story), into that structure is important for me and my listener.
I saw in Skern's form a structure that was like a relationship I had been in and the words came tumbling out easily, each verse was a different part in that relationship, each chorus the refrain of the idiot and fool that I was acting out.
So here ... the music came first and the melody and vocal line and words came together as different phrases of words with meter that fit Skern's bed of music popped into my head ... images ... I then threw them up and down on my tongue as I bounced a melody around those words ... driven by the bed of music.
Once I was actively into remembering the pain of this relationship, it just all came out easily. Got to know when to put the breaks on when that happens and remain focused on your songwriting intentions, as well as the techniques you want to use.
Keep your story plain and well defined, keep the images real ... in this case at least.
As to other songs, I have many folders with text files , some complete more or less, some just random snippets of lyrics that need music ... some with no melody at all in mind, some which can only be read by myself with exactly the same melody playing behind them ... and those will probably develop 'bed music' much more quickly.
Sometimes I find a few phrases in one folder, that go with another few phrases ... and I have another folder and I'm maybe that much closer to another song.
As to bed music for a song, that always happens for me either during the creation of the words and melody, or after I have a pretty good idea as to how I want the words metered and the melody they will be sung with.
By nature, I am a melody maker ... playing with words ... and I write music to 'accompany' those melodies and prose.
I have written bed music , like string quartets and the like ... but that's not what I'm into now.
I'm into song writing and performance.