Also....once your horse is good at handling ropes around the legs (up and down the legs) and stands still,...add one more step to be sure he is really good about it:
Ask him to move forward with one of the ropes around a front leg first, so he feels the rope around his leg with movement. It's different: movement vs stand still. Sometimes, a horse can be ok with ropes at a stand still then freak out all over again when movement is added.
Once the horse is ok walking around you in a circle with the rope going up and down the leg, add some slight pressure and ask the horse to yield to that pressure by stepping toward you (the pressure) with that one leg.
Then ask the horse to come to a complete stop and then remove the rope. So, the horse understands that all pressure is released only when he stops and stands quietly.
Repeat the above with all 4 legs.
This way, when you're riding and toss a rope and if he accidentily steps into it and feels it as he's moving, he won't care and will remain relaxed.