Rework an Existing Song
The blank page and silent guitar can be as intimidating for a seasoned songwriter as for a beginner. Writing a song requires a leap of faith that your emotions and ideas are valid and worth expressing, no matter what your inner critic has to say. Every writer learns by doing, and there is no learning without the doing.
Perhaps the best way to start writing songs, and to avoid the blank-page syndrome, is to mess around with an existing song. Make up new words to a simple tune, graft the words of one song onto a different melody, or improvise couplets in the pattern of jump-rope ditties and nursery rhymes. Forget about being clever or meaningful—narrate your daily routine, play with sound, anything to get the words and notes flowing.
Some musical forms are so embedded in our DNA that they make perfect launchpads for a beginning songwriter. Try writing your own 12-bar blues, for instance—all you need is a couple of repeating musical phrases. The primal Bo Diddley groove, too, is great for improvising rhymes. And the basic chord structures of songs like “Will the Circle Be Unbroken,” “Stand by Me,” and “Goodnight Irene” can accommodate endless numbers of new songs.
Excerpted from Songwriting Basics for Guitarists: Start Writing Songs