After watching videos, getting lots of advice and heaps of practice, I still didn't get a clear idea of the principles of reversing a caravan. So being a visual person, I worked out some diagrams to help us with steering whilst reversing. These seemed to help, so I thought I'd post them here in case they help others.
The main thing I found when trying to explain to my other half was that it's not a direct action- it's like when you play pool and you want a ball to go to the right, you bump it on the left side. So it's not a direct steering action, it's a nudge.
You end up with a snaking motion, and because it's not direct, you have to continually anticipate in advance and start steering your tow vehicle a couple of meters before you want the trailer to turn....then correct the other way well before you want the trailer to straighten up, etc.
If it goes awry, just pull forward again, line up and start reversing again.
Make sure your partner or "spotter" is clear on what instructions you want. Be careful of using "left" and "right", as the spotter's left and right will vary depending on which way he's facing. Distinct directions such as "Towards me" and "Away from me", with hand gestures, are useful.
The best tip I got was: put both hands at the bottom of the steering wheel - then depending on which way you want to reverse the trailer, bring that hand up (see diagram). Oh, and the main thing is to stay cool!
Step 1: Reversing to the RIGHT
If both hands are at the bottom of the steering wheel, RIGHT hand goes up, vehicle swings back to the left and nudges trailer around to the RIGHT. Allow time and room for the trailer to react.
Step 2: Reversing to the LEFT
If both hands are at the bottom of the steering wheel, LEFT hand goes up, vehicle swings back to the right and nudges trailer around to the LEFT. Allow time and room for the trailer to react.