Sometimes I will play a lick on the Martin and there will be certain words that just seem to fit. Sometimes while I'm writing lyrics the will create music that I start to hum. I will immediately sing the melody into my voice recorder on my computer or phone, singing the lyrics. Then later I will find where the melody fits on my guitar. Then other times I'm in bed or elsewhere and the music and words just start flowing out....run tape so I remember.
The point is, don't force the lyrics to music or vise versa. Just sit down at your piano and start playing with no sheets in front of you. make it up as you go. You may hear a run here or there that you really like and so it starts.