Actually it is quite simple. (Horses are all different of course, so each one will take differing amounts of time to learn.)
The way you can teach a horse to come to you in the pasture can be taught with a round pen. (Or any enclosure.) Later you will move onto bigger enclosures. You want to teach the horse that when he/she is near you, they get to rest and don't have to work. Over time, if you lunge them and maintain respect, they will catch onto that fact. Don't let the horse turn away from you when lunging, have them turn into you, and let them rest if they look at you/ walk to you.
This will translate (after a few days) into larger areas. It's hard to describe it on here (it would take forever) but that is the general idea. Don't try to chase them around the pasture because they can get away from you, and rest. You don't want them to be able to stop and rest without you.