Then I repeated the process and put in a trumpet part, which later turned into a flute and then into a violin (because I now have a better idea for what the trumpet will eventually be up to).
Again, in my mind's ear I heard a harp, so I laid it in there and suddenly the song had a beginning. But the beginning seemed a bit lacking, so I fooled around and added some tubular bells.
The piece seemed a little long for what had been done so far, so I tightened it up a lot, and then it seemed too crowded. So I looked at each part and realized that the cello could serve well to pick up from the violin and become a bridge to the next melodic passage. So the cello part is currently small and only towards the end of the piece.
I did put a quick ending (basically, taking the same door for the exit that was used on the way in) so it plays well as is, but like I say, it's just the beginning of something. If anyone is actually reading this (and interested), you should be able to monitor my (probably slow from here on) progress with the song if you choose to visit this page every so often.