This is one of the first things I will establish with a partnership. It involves straightness, landing in the corresponding spot from where you take off (not drifting left or right), quick reactions from the rider to prevent an inexperienced horse going off line.
The exercise I would normally use would be three uprights on angles to each other, generally at about 30 degrees to start with, building up to 40-45 degrees with the more experienced combinations.
I would generally work with a distance of 22ft between each rail from middle to middle. There should always be an obvious line through the centre, I normally use flower boxes or blocks and with an inexperienced combination I would use guiding poles on the approach, in between and on the getaway.
Start with the exercise very small before building up as there is a lot for the horse to take in. The first time they do the exercise they will often try to follow the angles of the rails so the rider must be aware and ready to ‘ channel or tunnel’ the horse straight.
Only when the horse understands and is confident should you raise the height, take away the guiding poles or make the angle more severe.