One way I have heard that works ok [havent done it yet !] that is with out ropes is to work them [get them sweat is best so they want to roll] & then walk them in the sand or where they like to roll & wait for them to get down & them praise them. You will have to put it with a signal too of course.
Little tricks that I have done to lay my horses down is by picking up one of the front hooves lift it all the way to the belly then get a treat and set it in front of him he will have to reach for that he will start to crouch as he get close to the ground let his leg go and he will lay down. With time you start to do signs so he knows what you mean and you can just lay and hang out with him!!! Good luck!