I did a lot of CTR for years. All horses have to be kept the same, If one has a corral, Then they all must have corrals Hence tying the horse to the trailer was usually the means of keeping the horse. tying to a high line was also equally acceptable, And I prefer it because I sleep better than feeling the horse tug at the trailer all night.
Trails has a good example of how to set up a high line, But the basic rules are 7feet high, 7 feet between horses and 17" of lead hanging down.
Get a suitable rope, I usually buy 50-60 foot lengths, Because they are a common length in the rope room of my local store. I can easily get 3 horses highline on a 50' rope and still have enough rope to fasten to the trees etc.
You will also need some kind of Tree saver to prevent rope burn on the tree bark. Old cinchs are great for this. But also a Nylon Web strap ( like a seat belt) works great. Place the Tree saver around the tree and attach the rope. I use a Bowline knot. Many beause I can get it UN-DONE after the horses have been tuggin and pulling on it. What ever knot you choose will be much tighter after 2 days of horses tugging on it.
On the opposite end, I usually use a rachet to rachet the line taught. But a truckers know works well.
On the line you use products like Knot Eleminators. But I usually just use some parachute cord tied with a Prussic Loop.
the big concern is you horse getting a leg up and over the lead while scratching his chin. Make sure you keep the lead fairly short, and make the horse stretch everything if he needs to reach the ground ( for hay or water.) I usually don't leave a bucket near the horse while on the high line, I offer water periodically, But if I leave the bucket the horse usually kicks it over and get a leg in the bucket.