The side reins should never be attached to any rings below the point of your horse's shoulder. The tension on the side reins should be slightly slack when your horse is standing at rest. The inside and outside reins should be adjusted to the same length to keep your horse straight on the path of the circle. Best suggestion? Spend a few dollars and get a used copy of the Pony Club manual off Amazon
(I prefer the British Horse Society version). All my students have a Pony Club manual as a reference guide (British version is one manual while the US version is on for every level).
Above advice is very important as well. Make sure you can lunge your horse quietly with contact on the longeline before you attempt to add side reins. No turning into the center of the circle while wearing side reins so make sure you can stop your horse out on the circle broadside to you.
Sorry this is so long but it is a huge question!