Depending on the ability and level of schooling of the horse, you can ask for several degrees of shoulder in. Shoulder fore, 30 degree, 3 track and 45 degree, 4 track.
The horse should be moving down the wall bent to the inside round your inside leg.
Imagine standing in front of a horse doing shoulder in, look at the tracks made by his feet. When he is going straight he is leaving two tracks with his feet. Back feet stepping in the prints of the front feet. In shoulder in you want to see 3 tracks. Outside back foot making one track, outside front and inside back making a single track leaving the inside front making its own track. Usually you want to be about 30 degrees to the wall. The hind feet to not cross over each other but the front feet will.
A more supple, schooled horse can do a 45 degree movement making 4 tracks. I imagine you are working towards a good 3 track shoulder in.
If you point your outside hip down the wall as you turn down the long side of the school, in the direction you want to go. Don't collapse the hip! Raise your inside hand to ask for the inside flex, Lower your outside hand and use the rien against his neck to ask him to move his forhand off the track. The inside leg swings with the horses' belly at the girth asking him to step well under with that hind leg on the inside. Keep your outside leg slightly behind the girth to keep him bending round your inside leg and to keep his backend from swinging out.