Make a template on the cardstock. Trace the template onto the wrong side of a scrap of fabric and Cut your circle out using scissors.
Thread the sewing needle with a double strand of thread and knot the end. Stitch along the outer edge about 1/8? from the edge. Pull the thread gently to gather the piece at the center.
Press the fabric gently with your hand to flatten and adjust the gathers as necessary to create a uniform circle.
Secure the flower by stitching through the gathers, in the center, 2-3 times. Make a knot at the end, and snip the thread.
Add some flair to yo-yo with a cute center by using a button and stitch that in place.