Shoulder-in is not as difficult as some riders think.
With the right preparation, balance, and logical aids, and by starting on a volte or circle, it is easy for the horse and a most useful training tool and remedy for common problems.
A natural remedy
According to La Guieriniere; shoulder-in should be the first and last exercise to teach a horse as it produces so many good effects. The first because it is the most natural, being the easiest position for the horse. The last because it remains the key to straightness and collection. As a training tool it is a great antidote for rushing, pacing, rearing, refusing to turn or to go forward (Shoulder-in can 'un-lock' the horse emotionally and physically). When a horse is in a panic or trying to escape we can quickly orient the activity to gain control and relaxation. Shoulder-in gives us a way to switch off a problem by redirecting the horse's balance and focus.
Start in-hand. It is useful to start on the lunge in a halter or cavesson, and then with work in-hand in a bridle, but this post is about pointers under saddle.
A point of focus
On the circle or volte it can help to focus on the middle of your circle (about 8m-15m) and imagine the front legs on an inner circle and the hind legs on an outer circle.
Shoulder-in on a circle requires the quarters to move around the shoulders, and the horse is balancing more to the inside foreleg. Laws of balance dictate that the rider sits in the direction of travel: to use the weight of her seat according to the direction in which the horse is moving, even if the bend remains the same throughout the lateral work sequence. Therefore for Shoulder-in on a circle - it's predominantly to the inside shoulder, (whereas for shoulder-in on the straight it's to the outside shoulder). The photo here is Alita and I working on a high, light and balanced position, from seat aids and posture (sitting tall, looking in, rotating the upper body in) asking her to mobilise and release the jaw to stay light and balanced, with inside flexion and soft rounding the poll. Alita had a tendency to lean on the hands, so we take care to remind her to carry herself and stay light and relaxed.
The hands give the position, the legs activate the quarters.
Guide the shoulders in first with neck reins (sideway rein actions), an opening inside rein to invite the shoulders in, a closing (or opening as needed) outside rein to direct the shoulders on the line of your circle. The important idea is to have equal contact in both reins to be able to guide the neck and shoulders at the same time - like in one piece - not just 'neck in'. Then activate the hind quarters for a reactive response to your isolated inside leg - long at the girth weighted - to create a pronounced crossing of the hind legs. (maybe a whip-tap on the quarters if needed). The circle shape can change easily with a loss of balance in either the shoulders or the hind quarters, so little adjustments to balance are needed - ask and soften, guide and allow, correct and release and as always, stop and praise for a good try.
If its not working
If this feels too difficult, you may find that counter shoulder-in along the wall (in walk or trot) should discourage the horse rushing forward, and help the sideway idea. If this is still difficult, its time to hop off and go back to the in-hand work to teach the shoulder-in on a circle from the ground. When you come back to the circle, place a cone or marker in the centre of you circle or volte to ensure you turn in to focus on it. This will help you do 'shoulder-in' in your body, to set the example for your horse.
Naturally one way is likely to be easier than the other. You will need to check if the issue is more you or the horse, or a bit of both. The best way is to get the video out and check on both sides. Check then the size of both circles if you can keep them the same. One way, the horse will offer more angle, the other way it will offer more flexion and bend in the neck.
From here you could progress to other exercises to improve balance and symmetry:
A figure of 8 around 2 markers, with shoulder-in on one circle into counter shoulder-in on the other circle. Keeping the same bend but not changing the size of the of circle. It's likely you find that one circle gets bigger and the other gets smaller, and when you change rein, it's the opposite. So, aim to keep them the same size to improve straightness and balance.
A figure of 8 with shoulder-in one circle and then single track, with natural position of the neck on the other circle. Keeping in the same bend.
A sausage type shape between two cones - where you do a half circle on one end in shoulder-in, and a half circle in counter bend on the other.
A straight line on the long side, into a volte in shoulder-in from the long side, back to the same point on the long side where the volte started. Remember to prepare before you take the circle.
Happy riding. I'd love to hear how you go. :)