Shoulder in is two tracking. You push your horses front over with your leg but keep the but where it was. It is easy to start doing this with a rail. Ask your horse to come off the rail with just his front. You apply leg at or in front of the girth. You do this both directions. You can also do a Hunches in which is the same thing except you push the but off the rail and leave the shoulders on. You apply pressure behind the girth.
A Half pass is side passing while going forward. It can be a little tricky to get down but it really makes your horse focus on you and work their body.
Start buy making sure they understand a side pass. Moving their body side ways with out going forwards. Then ask them to move forwards and apply leg to ask them to move side ways with out letting them bend their body. It is not easy at first but as the build their muscles and get more in tune to what you want it will get easier.
You can also work over ground poles to change things up and help with your distances.