The way I've done this is to supervise the horse and let it graze/walk around with the lead rope dragging. The horse soon learns to calmly take its hoof off the rope whenever he steps on it. He might panick a bit when he finds he is stuck but he'll settle down as he gets more and more used to it. Use a strong halter so it can't break - if it breaks it will teach him that pulling really hard is how he can get release. However, this should only be done while the horse is under your direct supervision otherwise he could get hurt. Especially if he decides to go for a canter with a lead dragging - not good!
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare