Getting a horse good out and about. The horse needs to have seen a scary thing and realise that it will be OK. So that the next time the horse sees the thing or is in the same situation, it knows it will be OK. It is that simple, you just need to get a horse through a scary situation without trauma. The horse doesn’t need to make super investigations, the horse doesn’t need to do fifty laps. The horse just needs to have enough exposure so that when the horse sees the same thing again the horse can say to itself ‘well last time this happened I turned out OK’.
A good example is dogs. A horse might be nervous around dogs, but normally once it sees one particular dog, it’ll work out pretty quickly that that particular dog is not a threat to it. If the horse gets past a few dogs without drama, it learns that dogs typically arn’t a big deal.