Hi Janet, we usually start our stock horses off in halter classes, because they have classes for all age groups and it is an easy way to get the babies out into the show ring and get them accustom to being at shows.
That means that our babies need to learn how to stand square at a young age, but you can still teach this to an older horse, it just takes time and patience. It is a simple idea of asking your horse to stand square every time it stops/halts. You help the horse by placing the leg/s that are out of place, it takes time but eventually they catch on and start to square up when they halt. And since they learn that this is the way to stop, it translates over to the saddle. Simple as that, not rocket science, just takes time on the ground.
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Courage is being scared to death but saddling up anyway. - John Wayne