The hardest part has been deciding what to reveal when. Being set in six different worlds there is an awful lot of information that it's necessary to get out there. This is why the average word count for children's fantasy novels is considerably higher than non-fantasy for the same age group. In my original draft I did the classic insecure writer thing of trying to get all the information in as soon as possible. I didn't trust my reader to work things out for themselves - I wanted to be sure that they completely understood how my worlds work. I've learned my lesson. Firstly, the reader does not need to understand everything about your world (or your characters or the background to the story...), they only need to know enough for the book to make sense and, secondly, people are intelligent enough to join the dots between what you tell them and what they need to know.
So, having written out a full time line of the book (and discovering that the action in the book takes place over 40 days and nights - how charmingly biblical), I wrote all the things that need to be revealed to the reader on individual scraps of paper and have then stuck them on to it at the appropriate point of revelation. This has allowed me to make sure everything gets said by the time the information is needed, but means I can spread all the "telling" out through the book without worrying I'm going to forget something. It's making the wall of my study look like something from Mission Control (see photo), which in a way, I suppose it is.
Having already written the first draft it has been really nice to plan the book again, taking into account the personalities of the characters and the unexpected twists and turns that happened along the way when I first wrote it. After several months away from it, it has been much easier to see its flaws and the places where things should be cut or rearranged, but also to find the bits that I'm actually quite proud of. Now, to pluck up the courage to write those most-dreaded of words: Chapter One.