The version I'm most familiar with, the one Woody Guthrie borrowed for The Ballad of Tom Joad, was a pick and strum bass line in C like this:

     C   F C G
  g c e f e d b a g ;
  
     C   F C G
  g c e f e c d ;
  
     C   F C G
  g c e f e d b a g ;
  
  G
  b b a e g a b b g
  
  G
  b b a b a g
  

(all notes on the bottom three strings of the guitar)