Reversing is a bit tricky, its a sort of balancing act.
The van always seems to want to go the opposite way from that which you are steering, getting used to this is the first stage of successful reversing.
So lets say you want to reverse around a fairly sharp right hand corner.
First pull up in a straight line leaving a reasonable distance - both widthways and lengthways from the corner.
Now turn the steering wheel about one turn to the left.
Slowly start to reverse - slow definately being the key word, carefully watching the caravan. After just two or three feet (thats why I said slowly!!) the caravan will have started to turn quite sharply to the right, hopefully in just the direction you want it to. Stop, and at this point turn the steering wheel to the right two turns - ie one turn back to straight on and one extra turn so that the car is now ready to turn to the right and follow the caravan around.
Inch backwards watching the caravan very closely (plus any obstacles around the front of the car too!)
The caravan will either start to turn more sharply ie, begin to jacknife, if so increase the amount of steering lock to bring it back in line - or it will start to straighten up - in which case reduce the steering lock.
If it is not going where you want and you are starting to jacknife, and maybe even full steering lock is not working, stop, straighten the cars steering, pull forward a couple of feet, (this will straighten the outfit up a bit, but not completely) turn the steering and try again.
It is all a balancing act - you are constantly watching the van to see which way it is trying to go and adjusting the steering accordingly.
But don't be downhearted! I consider myself fairly experienced in reversing, but even so I can rarely reverse a trailer without needing a shunt or two, but just pulling forward as little as a couple of feet can make it so much easier, so do not worry if you cannot do it in one go!