First things first, you have to be able to ride your horse straight before you can expect him to halt square. If he’s not travelling on two tracks and has a tendency to bring his quarters in, or doesn’t carry his head and neck in line with the middle of his chest, you’re more likely to end up with a wonky halt.
To begin with, practise riding straight lines using the edge of the arena to help you. a) Maintain an even feel down both reins. b) Rest your legs gently against your horse’s sides, creating a tunnel to keep him straight. c) His head and neck should be straight too, with his ears level. Now ride off the track and see if you can maintain his straightness. a) Try turning up the three-quarter or centre line and note what your horse feels like. Does it feel as if you’re drifting back to the track? If it does, correct him using your outside leg to keep him on the correct line. b) Make sure you’re looking up and ahead where you’re going, and keep your horse’s head and neck straight. c) As you ride this exercise, feel whether your horse is easier to keep straight on one rein than he is on the other.