Once you've wrapped the cardboard piece to your satisfaction, cut the yarn. Cut another piece of yarn about six inches long. This is going to form the center piece of your pom-pom.
Slide this piece of yarn into the middle of your cardboard piece, leaving one end hanging down on each side. If you're having trouble getting the yarn where it needs to go, thread it onto a tapestry needle and it will go right through.
Now that your thread is in place, all you have to do is tie it in a knot around the wrapped threads.
You should tie the pieces together on the side where the cardboard is cut, and tie the knot as tightly as you can because soon this is going to be the only thing holding your pom-pom together.