This is a very personal matter, so there are as many approaches to writing a song as there are personalities. I cannot tell anyone how he should do his stuff, but this is how I approach mine:
I always write chronologically.
Most of the songs I write start at the beginning. Once I have an intro that sets the mood, I do the verse, then I try to envision what I would like to hear after the verse and so on until the end. In doing so I follow the emotional curve of a song more like a creative listener, and less like the actual writer. I open my ears and try to hear how it goes on. That works a LOT better than my older approaches of throwing some ideas in roughly the same time and key together in the hope that they will make sense if connected smartly enough.
The other thing I learned about writing songs in this process is that your intuition can mislead you if you don’t know how to read it. I often find it hard to write vocal lines that really work, because not every melody you come up with is actually meant to be sung. Sometimes I mistake a line that might work perfectly in the background, played by some woodwinds, for a vocal melody in a chorus. That took me a long time to understand.
The last epiphany I have to share today is an uplifting thought about the ‘objective quality’ of a song that you write, i.e., the question if it is ‘a good song that people will like.’ today music is offered in such an endlessly pluralistic kaleidoscope of ranges and colours that it doesn’t really matter what you do, somebody will like it anyway. I somehow get the feeling that music was never less limited in style as it is now, with everybody doing their own stuff at home on a low budget. Making a living off of a song or a band is on a different page, but finding someone who appreciates it is not. Horay for pluralism!