OK. Well the first thing you would want to be certain of is that your horse is ok about the lines being around his legs and hind quarters. So check that out first.
One of the sneaky ways to start is to actually start off lunging with two reins, rather than actually long reining.
I long rein with a roller, with the lines going through D rings quite near the top - I just find it easier that way. But if you read Richard Maxwell's book or see him at work (he has a DVD out), he uses the outside line round behind the hocks.
I long rein at a "three quarter" position i.e. slightly behind and slightly to the inside of the horse. (This is when actually long reining, rather than lunging with 2 reins). I probably am closer to the horse than most people would be - you want to be close even to have control, but be out of the danger zone
Start with something quite simple i.e. straight lines and walk-halt halt-walk.
Build up from there with turns - you will find you need to release with the outside rein rather than tighten the inside.
Hold the reins as if you were riding, and aim to have a similar feel.
I use voice commands as well - so they get a verbal half half "....and...." then their name (if there is more than one of you in the school, they need to know who you are talking to
) "....Rosie..." and then the command "Walk On!"
I have a dressage whip held upright in the hand nearest the horse so I can just turn it down to tap their HQ gently if they don't respond.
You need to practice changing from side to side when you change rein if you long rein like this (and also changing the whip over).
Not everyone does it this way - I've just described what works for me.