You cut a circle piece of fabric double the size that you want your finished flower to be. Example if you want a 2in flower you cut a 4in circle. Turn circle so the wrong side of the fabric is facing you then fold over about 1/4 " hem around the circle. Sew all the way around the circle then pull..... It will bring the material down and give you the pleats. Stitch and tie it off. Finish with a button in the center sew on to a lined snap clip, then your all done. It's very easy. If that doesn't make any sense let me know and I will try to find pictures to show how to do it.
I found the link here it is How to Make a Yo-Yo: Step 01
p.s. Use no slip grip on your clip, because the weight of the yoyo flower will make it slip down the hair.