Complex sentence: They began work on the film 10 months before filming began, in order to plan out the Manhattan sequence, which was mainly CGI, though some New York location shots were used; the sequence was mainly handled by ILM's San Francisco and Vancouver offices.
Split into simpler sentences: They began work on the film 10 months before filming began, in order to plan out the Manhattan sequence, which was mainly CGI, though some New York location shots were used. The sequence was mainly handled by ILM's San Francisco and Vancouver offices.