Get an idea in your head to start, then throw it out on paper or an instrument then write it down if it is good enough to remember. If not write it down so you don't forget. Write lyrics or music whichever comes first. I have found myself personally writing most of the music, then I get a lyrical idea and create some more music to compliment the lines. Some of the best stuff can be horribly simple and horribly catchy, don't be affraid to be tied down by restrictions either. Let all the ideas come no matter how cheesey or lame they might seem. THough if something seems bad ass, go with it. I suppose it will depend on the mood of the song and the message of the vocals, and if you have none in mind, make it up as you go.

I've typed my peace,
later dude