@sophiejansonius 's answer is pretty complete so I'll just add a bit to hers. I recommend doing many transitions in that long and low frame, to make the horse more responsive and also helping him ship his weight to his back legs, making it easier to engage from there :)

Do lots of long and low, but keep the contact with the horse, and make sure that your horse is really moving forward into the contact, and not getting too on the forehand. From there on once you start to feel the impulsion and active movement, start bring your horses head up, but keeping the soft contact and nice forward active movement! Hope that helps :) :)