I'm subbing, because I'm not sure either.
The way I was taught - and this was a while ago when I had NO idea what I was doing, so don't quote me - is to use a fence line. It was something like walk down a fence line, and at some point, turn towards it a bit and pull back like you want the horse to stop. When they shift their weight to their hind end, use the reins to turn in towards the fence and start walking the other way. If done correctly, they will have shifted their weight to their hind end and pivoted to the other side when you ask for the turnaround. The reason for the fenceline is so when you do ask for the horse to turn the other way, they are forced to pivot instead of just walking in a circle.
I have never been able to manage this. Maybe once or twice, but it just doesn't work well for me. Maybe because I was a seriously green rider when I began trying it. It was at a four hour clinic hosted by my BO where Me and Clementine were the most novice - but we did well and actually Clem picked up on everything very fast. But the hind end pivoting... We just couldn't manage it. So, I'm going to see if anyone else has a better method for us to try :)