You have no problem sharpening from the shoulder to the start of the curve. That's great and will help you with the belly.
Now, as you sharpen, imagine a plane that exists relative to the blade, between the very edge to the spine portion of the blade. Let's say it exists at 20 degrees relative to the hone. In your mind, now extend this plane backwards from the spine for a couple of inches. This plane is what you swing the handle through as you sharpen the belly. It looks like you're raising the handle of the knife, but really what you're doing is keeping the very edge of the blade aligned with that plane. The knife must be aligned so that where the blade touches maintains that same 20 angle.
Now, a different thing that helps me. Forget the above and concentrate just on the very edge. Wherever it touches the stone, it must be going directly (90 degrees) into the stone. So as you push the blade forward against the stone, you move the knife handle so that point of the edge scrapes right into the stone. Now to add from the above paragraph, as you do this, the handle comes up in order to keep the necessary 20 degree angle of the edge into the stone.
For me as a right hander, as I sharpen the belly, the knife rotates counterclockwise, but the handle also swings through that 20 degree plane to keep the very edge at a consistent angle, from shoulder to tip.
Once you get the hang of this, you can improvise other techniques to suit your needs. But the above will get you sharp.
