It is so important that horses and riders are well trained over corner fences initially otherwise confidence issues can manifest.
To start with the building blocks need putting in place which consists of the horse staying straight on a line and being confident over an angled rail and the rider able to be in control of the quality of the canter and the line.
Using a corner coming out of a distance or double is quite advantageous to start with as they will be already on a stride pattern. I would always start with just the front rail to develop confidence and the line before adding the back rail and then the width. Once the combination can jump confidently through the line I would train them to approach either side of the first fence so they are jumping the corner independently and also on different angles (which they will come across in competition) instead of just at right angles.
Using corners on curving lines is a great way of teaching the rider to hold their line with their eye and not ‘waste’ any landing strides. It will also develop the riders feel for which direction the horse needs more help with.
Using short back rails in training is an excellent exercise as there is no room for error on the line and will encourage the rider to keep the line whilst in the air. It needs to be kept small at first as sometimes the horse doesn’t always read and understand the question to start with.