Start on the ground and whenever they back up say back and then do that for a little bit and then start doing it when you're on them and keep saying back and it should be memorize in their head
Apply pressure where at on the horse? @hannah
Start on the ground. First start with just applying pressure with your hands and release when they back. Once they get that concept, start including adding pressure with your reins. Once the horse understands the pressure on the bit and you know what ques they like on the ground, you can apply them when mounted. Sometimes you have to play around and see what ques your horse likes more. But once they get the hang of it all it should take it light leg pressure, and a small wiggle of both ring fingers to get your horse to back up.