The RYA Cruising Instructor Course is the entry level for any Yachtmaster who wants to begin teaching motorboating.
To pass this you will need to be able to give convincing demonstrations up to a Day Skipper level student of any part of the syllabus. Weak areas are often the knowledge of the collisions rules, navigation and meteorology.
There is no point in arriving for an instructor's course if you are not completely happy with your ability to teach tidal height calculations or working out the course to steer. Remember that your students may have just spent the last 5 days in the classroom perfecting their theory skills, some of them will be very good, even at Day skipper level. Anyone who is serious will do the necessary work prior to the course.
Another area that requires work is close quarters sailing skills, motoring on and off of buoys and returning to a floating object under power. I would expect a convincing demonstration of man over board recovery from any instructor.
It is recommended that Yachtmaster candidates have the latest edition of the cruising logbook and Instructor's Handbook.