@pomeroy Well, I think that it must begin with feeling (you will hear me say that a lot on here). When you feel sad, you play music which expresses your sadness, which means you will create the chord progression accordingly. Once you have the backbone of your song (either the rhythm or the melody) then you start adding the fillers (like bass, strings, etc.). This is my method. I don’t know how the others on this form does it.
If you want to create music for lyrics my best guess will be to determine what the lyrics are trying to tell. Basically determine the emotion the lyrics are aiming to convey. Then follow the process above.
:) My 2cents