The shoulder-in is where the horse is slightly bent around your leg but still moving forward. The horse walks on 3 separate tracks. One track having the inside-fore, another track having the outside-fore and inside-hind, and the last one having the outside-hind.
To start shoulder-in, go along the long side of the arena making sure both you and your horse are straight. Bend your horse towards the inside and apply your inside leg behind the girth and your outside leg on the girth.  Make sure you are not tilting your body to the inside otherwise your horse may lose balance and also make sure your horse is not just putting his neck into the inside, but having his feet on 3 separate tracks and actually bending his body.

Once you have mastered that on the long-side, try doing this on the centerline, this will make sure that neither of you are relying on the wall or track.