Shoulder in is a movement introduced in about second level. It is useful when the horse has established rhythm, relaxation and contact and will help to develop suppleness, straightness and collection when used properly.
I would not say that shoulder in is a basic exercise, I would call it intermediate because it requires carrying power from behind and a small degree of collection. Shoulder in, and any lateral movement should not be started on a horse until the end of their 4th year because of the physical strains.
Shoulder fore however, can be useful in correcting some issues when used sparingly and correctly.
Before teaching the shoulder-in you should comfortably be able to perform this (
First level test 4) with your horse. Everything should feel "easy".
Once you have established a good leg yeild where the horse understands sideways and a good lengthening and shortening of the stride in trot where the horse understands the leg aids for forward and back, then we begin the shoulder in.
At first, we want to keep a small angle, on four tracks (called a shoulder fore) Where in a mirror on the short side you can see all four legs. The horse should be bent to the inside, the neck should stay coming straight out from the shoulders and the inside eye should be visible. The inside leg stays at the girth to encourage bending, the outside leg comes back to keep the haunches traveling in a straight line and the rider turns slightly to the inside and uses her outside knee and elbow to bring the shoulders to the inside. The horse should not lose tempo, and if he loses balance a 10-12m circle should be performed to regain balance and bend. As the horse becomes stronger the angle can be increase to a three track schooling shoulder in, where the angle is such that the outside fore blocks the inside hind when viewed from the front. And finally the shoulder in is brought to four tracks at an increased angle. Lesson 15 - Starting Lateral Work: The Shoulder-in - Classical dressage