Complex sentence: Barks was a veteran Donald Duck artist when he came to pen "Nightmare" in 1949, having learned to develop Donald Duck plots and gags after joining the Disney studios in 1935, and collaborating with colleagues on Donald's first original comic book in 1942.
Split into simpler sentences: Barks was a veteran Donald Duck artist when he came to pen "Nightmare" in 1949. He learned to develop Donald Duck plots and gags after joining the Disney studios in 1935, and collaborated with colleagues on Donald's first original comic book in 1942.