Teaching the horse and rider to develop their eye and confidence for narrow fences is absolutely essential as at least 10% of most tracks are made up of either single skinnies, combinations of skinnies, skinnies on angles, skinnies out of water, etc. For me a horse has to almost look for flags and aim to go between them rather than around them so it is important to establish that within their training.
I generally use the ‘Jump for Joy’ telescopic wings with plastic clip on flags as I find them easy to move and adjust and they can take some serious hammer without being damaged, combined with short (90cm) poles. The great thing about these fences is that they also go in my car so they can be used at home on a surface or I often take them with me whilst xc schooling to use after drop fences or coming out of water, etc.
I would always start with the horse being walked quietly through the wings with no pole so they understand the concept of going through narrow wings. I would also repeat this a few times in trot and canter with lots of praise for the horse. Get the rider to focus on the straightness before and after the wings so the horse learns to stay straight. When confident add the pole (with some guiding poles if very wobbly or inexperienced) and practice jumping from both reins and directions.
Moving on from a single skinny I will normally practice curving lines, straight related distances, across a short line with limited approach strides, as the middle element of a combination or at the end of line of show jumps to test focus and straightness. I would also make skinny fences a regular occurrence when jumping so horses get used to reading them.
I will occasionally use fences without flags to develop the riders skill of maintaining straightness in the air and on landing. For this exercise I will set up some small, narrow fillers and ask riders to trot and pop them. Initially the horse may try to look for a way around them but if the rider holds their line on the take off, mid flight and landing it soon becomes a fun exercise for all levels. It is especially good for the rider that abandons their horse on the approach as they start to take more responsibility.