The casting is the easy part. Its the explanations that most find difficult.
It is not a casting examination - so dont fall into that trap. It is a teaching qualification, so learn to explain each aspect of the cast in clear, concise wording. Less is more - dont use 20 words when 10 will do.

Have a good understanding of the fundamentals of fly casting. Bill Gammels 5 essentials should be learnt, understood and played with to see what effect they have on your casting.
This is important for when sitting the written.

Best of luck - its a learning experience!