The first cue for asking for the rein back on the ground is to stand facing their tail, while standing at their shoulder. You hand hand closest to the horse's head should be settled on the knot of the halter. From there, lift your energy and intention and apply a little pressure on the halter towards the hind end of the horse. If the horse doesn't respond or pick up on the cue, you can add a little driving pressure by gently tapping on their shoulder until their energy starts to move backwards.
When teaching the rein back, make sure to give them plenty of time and only ask for tiny steps in the beginning until they have understanding and confidence.