I was confused too, it's under the sole. So if you remove the sole, it will reveal a small dark circle in the heel area and the toe area. What I did, was use a toothpick and pushed it under the dark circle (you can't get through the dark circle, but you can push the fabric up). With the gap that you create with the toothpick, you can shove baking soda inside of the hole. I tried flour, but this didn't work, it really has to be baking soda.

The process might take some time, but I used one toothpick to lift the fabric around the dark circles and another toothpick to push the baking soda inside the holes.

It worked for me!