Thank you for your question!
1) For the leg yielding, your inside leg should be slightly backward, your body should move in the direction you want to go (but you have to stay center in the saddle, it is more your energy that should go in the direction you want to go), and your hands should be as quite as possible. The worst would be to move your hands to the outside (it happens when the horse doesn't react enough to your inside leg). You should feel the will to move your hands slightly to the inside.
It is not completely true but I like to explain that your inside leg should move the whole body of the horse to the side and your outside rein controls the shoulders.
If the horse move too much to the side and not enough forward, you use your outside leg as if you wanted to go straight forward, it usually helps to avoid the horse falling sideways.
If the horse moves too much forward and not enough sideways, sit a little bit deeper in the saddle and imagine you want to go backward.
2) For the shoulder in, the main difference is that the whole body of the horse should be bent around your inside leg like a banana. Again, it is not completely true because some parts of the horse's body cannot bend, but if you keep that image in your mind it will work better. So here your inside leg should be at the girth. And your hands should come slightly to the inside. The outside leg control the hind legs if the horse gives too much angle.
Do not hesitate to keep your outside leg in front if you feel that the shoulders don't really comme inside. Usually trainers say that your outside leg should be to the back but in my opinion this is only true if you have too much angle. If you have a problem with the shoulders, use your outside leg in front to support the action of the outside rein.
Don't forget that the name says it well: it is shoulder in and not haunches out. So don't push the hind legs outside, really feel that you bring the shoulders inside.
And if you feel that the horse stays stuck to the wall, you can open a little bit more the inside rein, it usually helps, but don't forget to bring it back to its normal position as soon as the positioning of the horse's body is good.
3) For the half-passes, inside leg in front to control the bending and the shoulders, outside leg backward to push the hind legs. Contact (control is better) on the outside rein and inside rein supporting the inside leg to get the bending.
This is the very short version because it usually take a lot of pages to explain well!
Think about these exercises like this:
- Leg yielding is to teach the horse to move sideways without getting crooked. For them it is easier in the beginning to get crooked than to cross their legs. It is also a good exercise to get them looser in their body.
- Shoulder in teach them to bend around your inside leg and to look for the control of the outside rein. It improves balance and make them stronger and more collected.
- Half-passes already need a certain degree of collection, and improve the respect to the inside leg. You must have a good response to the inside leg before you try half-passes.
About how to start, it depends a lot on the natural tendency of the horse.
I like to start with counter shoulder in with inexperienced riders and horses (so shoulder in facing the wall) because the wall will help to control the forward motion. Then both the rider and the horse can concentrate on moving sideways without dealing with the tendency of the horse to rush forward to avoid the effort. This way, you avoid the rider pulling on the reins and the horse to go against the rider's aids.
After that, I like to start leg yielding in walk. I start on a center line, stop the horse, and ask him to do one or two steps in leg yielding then stop and reward if it was good.
These exercise helps both the rider and the horse to understand quickly the importance of the outside rein to control the forward motion.
Of course, if I see that the horse wants to move too much to the side, I will take a longer diagonale and will not ask the horse to stop. Instead, I will ask a few steps sideways then move forward on a straight line parallel to the center line.
When leg yieldings are good enough, I will try the shoulder in, usually from a volt so I already have the bending. The shoulder in is the first step of a volt that continues along the long side.
I hope this is clear as a starting point! If you want more informations on these exercises, I recommend that you read "Dressage with Kyra" from Kyra Kirklund where it is very well explained.