My two cents, I do it pretty much just as mscamp does and how cowpunk'e described with a little difference. I give the horse 3 to 5 ft of line and stand behind the shoulder and always in front of the hip. With the line in hand I extend my arm in the direction I want the horse to move out (I should say, your horse has to be halter trained before this). Then with a lounge whip in my other hand I slowly "flick" or roll my wrist so the end of the whip gentle rolls out on the ground behind the horse, giving the horse the verbal que to move out. Try to keep your horse in a walk or slow jog, short lines (tight circles) are hard on legs, don't scare you horse, keep it calm, that's why a gentle rolling out of the whip on the ground works better than popping the whip (also, you may have to touch the horse with the whip to encourage it to move out, roll the whip out onto the horses hip or back legs). Just as Cowpunk'd said to stop give the verbal comand and step toward the head. Don't allow your horse to come to you without giving it the command, if you do it becomes a problem to fix later. Some like the horse to stop and stand in spot, others like the horse to stop and face you. I have them stand in spot until I ask for them. Be careful of young legs and circles. Ms has already told you how long to work your horse, always quit the training session on a positive note. Even if it's 15 minutes of frustration and 3 steps in the right direction good place to stop and reward him/her.
I usally start longe line training at about 6 months for 5 to 10 minutes a day. I only do 5 to 10 minutes because of young legs and youngsters only have about a 5 minute attention span.
Ms and cowpunk'd do their training very close to what I do, you'll find with horse training there are many ways to get to the same spot. The biggest keys are to have alot of patience, don't scare your horse and always end on a postive note with a reward (I rub or scratch the horses neck or hip or ect. I don't like giving food treats during training...but that's a whole other can of worms).
Good luck,
Alan