I have gone through this repeatedly with my drivers school students. I would suggest that (as I suggest to my students) that you know two things. You should know:
The flags, what they mean AND what your responsibilty and actions should be.
This is to my mind is the MOST important thing.
The second thing that you should know is the "rules of the road" in the GCR. You should know what the rules are and what your responsibilities are.

If you know those items you can then treat the GCR as a reference as it should be treated.

cheers
dave parker