Complex sentence: Carradine worked very closely with his friend, singer-songwriter-guitarist, Guthrie Thomas, on the Bound for Glory film assisting Carradine in the guitar style of the period and the songs that had been chosen to be in the film itself.
Split into simpler sentences: Carradine worked very closely with his friend, singer-songwriter-guitarist Guthrie Thomas, on the "Bound for Glory" film. Thomas assisted Carradine in the guitar style of the period and the songs that had been chosen to be in the film itself.