A round pen( no rope). Have them walk, trot, canter, stop, transition down on your cue. And so the same thing the other way. Make sure that when they are reversing they face you and go the other way on your cue. When they decide to turn around without you asking make it your idea and make them turn around and run a few steps then turn around again and run a few steps. Usually you have to be fairly forward(not aggressive or abusive) and make them listen to your cues. I train 2-3 year olds to lunge a lot. This technique is amazing and they learn to reflect you very quickly. The part where they face you when they turn around is then respecting you and I'd bet important that they learn that. It they don't turn towards you to reverse make them turn around and run a few steps a few times and keep at it. This usually takes about 20-30 days and then use a line a couple times before taking them out of the round pen to lunge
A 60' round pen is ideal when teaching young horses. If not, it's a good idea to teach a young horse boundaries and to give to pressure. That way, they catch on faster when you send them off.