Complex sentence: The amount of rendering caused the cels to buckle, which made it difficult to keep them aligned for photographing; assistant John Fitzsimmons developed a technique using a loose-leaf binder with the center cut out to frame the cels firmly in place for tracing and shooting.
Split into simpler sentences: The amount of rendering caused the cels to buckle, which made it difficult to keep them aligned for photographing. Assistant John Fitzsimmons developed a technique using a loose-leaf binder with the center punched out, which was used to frame the cels firmly in place for tracing and shooting.