There Are Many Approaches
I first learned to long line over 25 years ago from Lockie Richards. He taught me the basics of driving my horse with me behind as a way of introducing the piaffe. My horse never did quite master the movement, but I found lining a great way of encouraging him to go correctly on the bit and began to line instead of lunge soon after.
Some people call it "double lunging" as you are using to lines instead of one. The advantage to lunging, in my mind, is that the contact to the bit is not static as it is with side reins. However, it also demands that the handler think a whole lot more about how to work the reins.
I don't much any more walk behind my horses, mostly because my bad knees make it hard for me to run so we can only work at the walk. That's fine for some of the lateral work and introducing piaffe, but it's hard to do much else. Also, because the walk it the least "impulsive" gait, getting the right contact to the bit can be tricky unless the horse just naturally swings along in a good free walk taking the reins. (My guys like to get a bit lazy.)
I've used lots of rein configurations but prefer attaching the lines directly to the bit, then running them through the top rings of the surcingle to my hands. This is most like the postion the reins would be if I were riding. I run the outside line over the horse's back, not around the hindquarters. I don't like using the line around the rump because it can get caught in the tail and also, should it go slack I think it's a potential danger to get caught up in the hind legs.
I know plenty of experienced trainers put the lines lower and use them around the rump, but I am just not quick enough to correct things should we have a problem--again with the knees--so I like to take a more conservative approach.
If I do need to drop a line to a lower hole, I will most likely just drop the inside line.
Another option is to "vee" the lines by fastening the line to a lower dee, running it through the bit and up through the top dee to my hand. This gives a "draw" rein effect and far more leverage on a horse that might be really resistant to giving to the bit. However, again, you have to be really careful in using your hands as it is also very easy to get a horse overbent--into the dreaded rolkur--if you don't "give" the lines as soon as the horse yields. It's tricky.
I use the "vee" technique on Chance now and then to get him to go down as he likes to "stargaze" when he first starts out. Then, I am very light once he gives because when he gets the idea, he is soft. Toby can be tough sometimes so I might use it more. It is not the best solution for Tucker as he will overbend very quickly.
The outside line is always the key, just as the outside rein is the key when you ride. It is the line that brings the horse onto the bit and essentially controls the "height" of his head--the frame. The inside rein offers direction and bend, but really does need to work with the outside rein to get much done.
Aside from my start with Lockie, I have never had any other formal training on using the long lines and have figured out most of this by myself. I have had more than my share of mishaps including loose horses galloping around the ring with the lines trailing, and one of my favorites, horses who have spun around to end up wrapped in the lines.
However, when the lining works, it is a wonderful training tool. When Toby was first ridden after having been lined for a year, the girl who worked him for me kept insisting he was a trained horse instead of an "unbroke" three year old. She had him cantering after the second ride and he was steering and stopping like a pro. Tucker was largely started on the lines as well and when I first rode him, he stopped and steered well too. Chance loves trails so much I tend to do that with him more than the lining, but this past spring, I focused on lining him for about two weeks and it made a huge difference. (He had a injury soon after and was laid up almost all summer, so I never did finish up that effort.)
I've jumped my guys in the lines, and worked them over cavaletti. I've tried flying changes with little success, but the Spanish Riding School trainers have no trouble with that. From what I have seen, a good trainer can do most everything a rider can do under saddle in the lines. I'll never quite master all of that, but who says I won't keep trying?
All in all, I have found long lining to be a great alternative to riding, and, as I said in my last post, a good way to actually see your horse in action.