Thread a needle with matching thread (I used a contrast here only for instruction) and knot.(aside: I know it's a small thing, but aren't the little things what makes life fun? I have found that tying a perfect knot is so much easier and more fun now that I know how to do it using Heather Bailey's tutorial found here )
Begin in the center.
Insert your needle into the center dot on the stay and pull up through the center dot on the larger square. Take a stitch back through the center, a couple threads away from where you brought the needle up, and then back through the center dot on the stay, also a few threads over from the first stitch.
Take another stitch through the center to anchor the larger piece down, and repeat at each marking. (second photo)