Complex sentence: Powell wrote a book about his experience making the film: raising the initial funding, trying and failing to make the film on St. Kilda, then realising that Foula could be used instead.
Split into simpler sentences: Powell wrote a book about his experience making the film. How he raised the initial funding, how he tried and failed to make the film on St. Kilda, how he realised that Foula could be used instead.