I usually start strumming my guitar to whatever emotion I am feeling at the moment. If something seems to feel right, I may say a few words that come to mind. Often those words are meaningless, but seem to help support the emotion of the song. Slowly, the real feeling and direction and subject of the song starts to gel.
I find it extremely helpful to have an easy to use audio recorder to capture even the most non-sense of playing/words. By easy to use I mean NOT one of those all in one video-audio-game- players that do everything. That's too tedious. I use an olympus voice recorder - DM-520 which will record in lossless format. But it's biggest advantage is that the controls are physical buttons that can be controlled by feel. And so I can record in small takes easily.
Sometimes if I like what I am hearing and can't seem to take it far, I put it aside for a few days. That's where the audio record is helpful, I can talk out chords so its easier to remember later.
I think of writing music as having a daydream. One where you are in partial control, but not all.