What you do is paint a picture with the gouache. Leave the white of the paper in any areas you want to be black. So it is a good idea to sketch out your idea first lightly in pencil and draw around the areas you want to be black. What happens is after your gouache painting is completely dry (overnight) you take a large, soft watercolor brush and cover the entire thing in an even coat of black India ink. After the ink is dry (I gave it a few hours), you rinse the painting under running water and, using the soft watercolor brush, wash away the ink. The ink will stick to the white paper but the gouache repels the ink so it washes off and the painting underneath is revealed. Just keep in mind gouache is water soluble and some will wash off with the ink. Use the paint thickly, because if you do thin coats, it could all just come off. In the 2nd example I did of my shoes, I didn't use the gouache thick enough and a lot of the floor and skin tone area came off with the rinsing.
I just realized maybe I should have put up the photos step by step as I did it, but these were already done before I had the idea to do this post. Next time I do this, I will take pictures and document the process. It's really fun, give it a try!