Agreed lots of ground and trust must be established. Agreed with the others to do this you need a solid well broke horse which can give confidence to the green horse. Also need a well established role as a leader. I'd start with good respect work and join up exercises I'll post a video of me working with the colts and asking them to join up and follow at liberty. I use my body to draw them in I don't look at them in the eyes when I do look it's indirectly good places to focus your eyes are the neck,chest, or legs. I point my shoulder at their face/nose I reach my arm outward but lowered with a neutral hand. I did a little work before this to make my hand a bit or a target of interest it helps draw their attention when I want it. When I have their attention I will walk a few steps usually somewhat toward them or past them sometimes you have to ask a few times. Also I send them away and redraw their attention when they turn their back and rear to me I expect them to face me. Also Jet is usually more interested in me than Jackpot so I can often use Jet's body(the bay) to influence the movement I want in Jackpot (Dunskin) when I don't have his full attention I image the people that control on those horses at liberty must do something similar. When they come to me I reward with lots of praise you can build more and more on this as you get better. My adult horses follow me around everywhere now with much reasking even in crowded places. Just recently I've started working on doing what you want to accomplish its had its trials and errors but I'm starting to get ahold of it. Just the other day I took my horses out to the arena my friends had their 3 year old daughter with them I put her on Pepper while I rode Joy she just held on to the saddle horn and Pepper just followed me and Joy as we rode around the arena he could of easily wandered off if he wanted to but he stayed even right with me even when we trotted a little. So keep at it you can get there it just takes a lot of time and practice
Click links for videos let me know if I need to explain the body language any better I will certainly try. I hope they help you.
PS: this is also great for catching your horses even when they don't want to be caught.
Join up and follow Jet and Jackpot F0013A60-0C86-46B8-845D-17DDC9F8F808-2215-0000027B5C738A5A.mp4 video by catnoles | Photobucket
Playing with them at liberty to get them to walk over obstacle together D09A38D9-EBA5-42D2-9A59-2E5A97507D61-5280-000003B7E499E3C8-1.mp4 video by catnoles | Photobucket