Hey Michele:
Gosh, you've gotten a lot of great advice here!
I had one more thought, that was a pretty clear step that helped me start to see how lateral movement could get shaped into things like shoulder in...
You could start with the target just working to get her to walk around you in a volte (a fairly small circle).
So, if you want to get her turning around you to the left, for example, you could:
1. Hold the target in your left hand.
2. As she starts to follow the target, turn your body so you're turning backwards on yourself to the left.
3. Walk her around you so she's completed a full circle.
As you get comfortable with this, start to watch where she puts her head, how her neck bends, how her shoulders follow her neck, how she bends through her body, and then, lastly where her feet are landing.
The first thing you can work for is getting a nice round bend as she's circling around you.
A shoulder in basically is asking the horse to stay on this bend as she walks forward -- she stays curved as she walks forward.
I found this was really helpful to be able to see even before I began cueing her hind to cross under (which is another great step to work on the circle, even before you work for the shoulder in -- after you experiment with some of the cueing that Karen and others have suggested to start to get Chance to step under herself).
One of the GREAT things about beginning to explore this from the ground is that it's much easier to actually see the movement you're asking for. I rode lateral movements for a number of years before I did this from the ground and I understand what I'm going for so much more clearly being able to see it! (It's hard to see when you're riding -- you really have to go by feel alone!)
Another thing that helps me is to pretend I'm the horse! I've spent a fair amount of time walking in circles trying to get a sense of the directions my body would go if I'm doing lateral movement.
Hope this helps!
Best,
Leigh