The material you need is 8 feet of 1/2 or 5/8s marine rope. It is soft but strong. A good brass snap, I hate the steel ones and brass is actually stronger. One ring about 1 1/2 diameter plus about 10 feet of 1/8th strong cord.
Burn the ends of the rope to keep them from fraying.
Double one end of the rope through the snap and fold about 4 inches back along itself. Then using the 1/8 th cord wrap it around the doubled over end. You decide how to knot it. I have an invisible way fo doing it, a hangmans knot but most will not know how to do this and I can't explain it here.
I then paint laquer , varnish or anything else over the string to harden it and prevent it from every moving. Anything that hardens just makes for a nicer finish but not necessary.
You then just pass the other end through the ring and tie a knot where ever it makes the right loop to go around your horses neck but not oo tight to make using the neck rope a pain to snap in.
I don't retrieve the horse from the pasture with a halter, I just use the neck rope for just about everything without the halter exoept for tying. Use a halter for this.
Post any questions and I will answer them later. I am heading out for a ride