The picture shows three embedded squares, the smallest is IJKL (green), the medium EFGH (blue) and the large ABCD (red). You can resize the squares to make the design. Shade the green square (for example, colour it green), and shade the outer area between the red and blue squares. That leaves an unshaded area between the green and blue squares. I hope this is what you were looking for. The picture shows a symmetrical design but it doesn't have to be symmetrical. By moving the green square about inside the blue square, and then move them together inside the red square you can get many different designs.