tldr; but this is pretty much my workflow on 90% of my stuff
Initial creation
I also start with a song in my head, not something I try to think up - it just appears there. Usually I have the drums/bass with some chords and melody just keep popping into my head, but usually only about 16 bars worth.
After a few days I notice that I have been "mentally singing" this to myself and then I check to see if I have already made this song, or whether somebody else has. If it's actually fresh then I will start laying it down in Session, just those 16 or 32 bars as I heard them in my head. Often i wont try to get the instruments to sound the same as I 'imagine' them, I just lay down a quick approximation. Once the rhythm and riffs are down I do a bit of sound design to get the sounds to be more like I imagined them initially. usually I am then coming to the end of that burst of creativity, life calls, so I record a 'jam' of the session to arrangement to help me remember.
arranging
next time - I play back the rough jam and something usually leaps into my mind, a flash of "it's obvious!" ...an immediate understanding of what the arrangement should really be. Like looking at a half worked on sculpture - I can squint and see what is suggested. IE: usually what the vibe of this track will be in its entirety and what the wider instrumentation should be and how the 'story' of the track should play out. Where the tension should be in the melodies, and what it should be 'saying'. If this does not happen, if nothing suggests itself to me I assume the piece is a dead duck and it goes into a folder of doom.
However - if it is evocative I will start to construct the other sections as I imagined them, programming parts and doing the sound design for the other instruments and parts, etc.
step away from computer - receive inspiration
At this point it is 1.30 in the morning, I turn off Live I go downstairs and make a cup of tea and immediately am struck by inspiration of what is missing in the track. It's pressingly obvious, a counter melody, or a harmonic shift...damn it ! So I then run back up and turn Live back on and quickly add that part. It is now 5am in the morning.
repeat this process until 3 weeks later the song is nearly finished then get a huge amount of real & paying work in that takes me away from the track for two weeks.
some time later
listen back to the track, notice that the bassline is out of tune, notice that I could probably just retrack half of the parts because I now now what I was trying to do. Retrack some of the parts.
Final stages
become convinced that the track is finished. render it out with a name like "temporary song title- final"listen to the render, notice something that needs fixing
repeat these "final stages " about 100 times, ending with a filename something like "temp song - final final June 2011-k final"
play render on as many setups as possible, making notes.
if necessary go back and edit sounds and or arrangement.
re-render a further 40 times.
song is now finished