Honestly, if you have “that” horse, it may be something you are stuck with… much like my “can stand on a lead rope 50 times a minute and not freak out but god forbid he hits the end of a rope he is tied on” asshole, er, I mean beloved retiree.
But what I generally do is start with a very short rope with a sturdy snap (one that can’t get wrapped around a leg but long enough to actually be stepped on) and a halter that will not break (if you use a rope halter make sure you know how to tie it in a way that doesn’t tighten the knot). Turn them loose in a small controlled area (round pen preferably, stall is not ideal, but OK). Give them hay, maybe in a couple spots so they have a need to move around. Find something that occupies your time in the vicinity for a good long time, depending on how special your horse is. Take apart and clean all your tack! Ignore them, if they freak and you immediately run up there it probably just reinforces the idea it really was scary after all. It’s not pretty with some of them, but even if the horse is always going to be DRAMATIC!!!11!1! about the pressure, at least they know it isn’t really terrifying.
If they don’t get past this without giving up being dramatic, I’m not sure I would bother going on, but at least you know you have done whatever can be done to make them less reactive. If they do get past it like most horses, halter, long lead rope, out in pasture or a dry lot. Keep an eye on them, but mostly they are just dealing with it at this point. I have a bunch of 6’ lead ropes that are perfect for this, not long enough to get into trouble with, long enough to step on repeatedly, they are also thick enough that they don’t wrap easily. If you are worried about a lead rope being too bendy and wrapping around a leg, just get some thinnish strong rope and run it though a length of old garden hose - with a knot at either end. That’s an old trick from Ye Olden Days to stake out a horse so it can’t easily get wrapped up while staked out.
The last stage is getting them used to a long rope following them. For me that’s just part of ground driving. I want a round pen for this but if I trust the horse, I’ll do it in a small arena. I always start with just two lines and regular lunging. One line is normal, the other is crosses over the withers and is just “there”. When we halt I work on accepting the feel of that line on the back and over the butt. If that goes well, I use it to ask for a change of direction into a halt. Once that is solid in both directions we work on some very slow controlled changes of direction at a walk with a lot of halts.
Finally I work up to trotting with the butt rope. If you have a fjeral fjord, it’s about this time he makes progressively larger and faster trot circles, and since you don’t have a round pen, the next thing you find out is that he is totally fine with 20 feet of ropes trailing after him on both sides after about 12 energetic trot steps, and you also learn he comes back to you when he is done self lunging. But Fjords are different.
But back to normal horses. At some point when I know they are A-OK with ground driving and ropes that may be on their butt, hocks and even under the tail, that’s when I want to introduce them to ropes that drag along behind the because I’m shooting for a horse who has a career in dragging things behind him! Me walking him with that inside long line and the outside one crossed over his withers and on the same side is a good start. All inside turns and ONLY inside turns. Do the other side as well. But again, only inside turns meaning that rope is always on his inside eye.
When it is time to do outside turns, be prepared, those are always much scarier. Since you probably don’t have breeching with trace hangers handy, again I would leave that rope going from left halter ring, across the withers over to the right side - that gives a feeling of being back across the haunches and behind them without being easily stepped on. As you are on the left do some straight lines, then make a left turn, wide and slow if you have a special child. That’s usually the moment the rubber hits the road, they can hear the thing “chasing” them but they can no longer see it. So the instinct is to whip around and face it, which of course makes it once again disappear from view. This part can get real fun with some horses said nobody ever. But most are pretty cool about it and if you have done all the steps carefully, you will KNOW if you have that kind of horse and you can figure out if continuing on is in your skill set. Your job is to anticipate the above and shut it down before it builds. If the horse is just a bit nervous you can go back to straight lines and try again and see if he is willing to listen to you. Most of the nervous ones are fine once they figure out that you think it is no big deal. But there are a few who tell you in no uncertain terms that they do not intend to have a career dragging things around. But since it is snapped to the same side you are on, if things go sideways, as soon as possible, you can unsnap that drag line without having to cross in front of a potentially volatile horse. If they continue to freak out, the drag line will just slide off their back and be gone in a second. If they are fine I work up to walking and trotting, both directions with that line crossed over (or in trace hangers if available). That way I know if something ever goes sideways, the one thing I’m not worried about is the horse dragging his lunge line or harness and reins across hill and dale. I mean he may still be panicking for a thousand reasons horses dream of i their sleep, but the equipment isn’t adding to the problem.
Then if you are really a masochist, you can work up to attaching milk jugs with some gravel to the end of the drag line. 
Then at some point you may find yourself buying a few carriages and having more fun than should legally be allowed.
