Doing the simple change towards the wall at A (or C) means that the physical barrier helps the horse to realise it needs to wait and ‘sit’.
Doing the transition in the same place repeatedly also helps with this sort of exercise – most horses will start to anticipate the transition and you can praise them when they willingly shift their weight back ready to go to walk. This means you can gradually refine your 1/2 halts, making them lighter and lighter.
Try to do as much of the turning on the circle as you can with your outside aids, particularly your outside rein. The feel should be of bringing the horse around rather than of steering with the inside rein.
This is hard work, so I wouldn’t do too many until the horse is fairly fit and used to this level of exertion/collection!
This exercise can of course be started on either rein.