Focus on it for a few days and play with different melodies in your head - especially going to sleep.
Hopefully something will pop up that is catchy.
Once you have a catchy melodic hook then you can develop the melody and words a little at a time- it doesnt really matter in which order.
Once you have a core phrase or tune that you can't get out of your head - a lot of other stuff seems to just flow from it. Its like the hook is the seed - and if the seed is good enough - it will do whatever it has to in order not to die in your head.
this is a process that seems to work for me.
