The bend of the horses body around your inside leg helps improve the suppleness laterally, the bend should be equal from nose to tail. The movement is ridden from your inside leg which holds the horse and stops it turning across the school and the inside leg which directs the shoulder in and off the track.
When starting, it's much easier to use the wall or fence of the arena to help you. As your horse gets more established, you can ride it on the centre line or round a circle.
As you ride the shoulder in, think of being able to alter the angle through the ribs, the flexion through the neck and the tempo to develop the suppleness further. Change something every time you ride a shoulder in - ask for slightly more flexion, or ride it in a more medium trot for example and practice being able to move every part of the horse around.
Top tip - ensure the horses shoulder is moving IN and not that the quarters are moving OUT.