Cut the CDs into small squares using your sharpest pair of scissors. This step is quite difficult as I found that the reflective paper on the CDs tended to peel/flake off as I was cutting them.
I found a few techniques online that are supposed to make cutting the CDs easier. These included: placing them in a bowl of boiling water, heating them with a heat tool or hair dryer or brushing a layer of PVA glue over the top of the non-reflective side of the CD. I tried all of these and found the hair dryer to be the most helpful (the heat makes the CDs more flexible and easier to cut). Be careful with this step though, obviously with the heat from the hair dryer, but also the CDs can be quite sharp once cut.