Original: It was directed by Franklin J. Schaffner from a script by Francis Ford Coppola and Edmund H. North, who based their screenplay on the biography Patton: Ordeal and Triumph by Ladislas Farago and Omar N. Bradley's memoir A Soldier's Story.
Simplified: The movie's screenplay was based on a well-written biography of General Patton, Patton: Ordeal and Triumph, and A Soldier's Story, a memoir by General Omar N. Bradley, who served with Patton.