Try this exercise. Sit seiza and have someone push down with both hands from behind on your shoulders. Breathe in slowly through your nose, expanding your diaphragm downwards. Imagine your core is becoming denser and your body inflates from the center. When you exhale slowly from your mouth, allow your diaphragm to naturally rise upwards. Imagine your core is becoming lighter and you feel a firmness throughout your surface area. When you breathe in, the person pushing should feel a sense of pneumatic expansion and when you exhale, the person should feel a sense of being led to your center. Let me know how it goes.
This is a great drill to start to get the "mind/intent leading the ki" piece going as well as using the breath to start to connect the whole body together (don't do too much too fast at first as there's some risks there, best is someone can show you). Add being able to connect centers and direct ground/gravity (earth/heaven) into somebody using the same intent/mind leading and you've got some of the very basics/foot in the door stuffs to IS.