2. Two lacrosse balls. They can be used or new. I suppose tennis balls could be used, but they are a bit bigger.
3. Cardboard. An old cereal box is perfect. We'll use this to make a mandrel to tie the knot around.
4. Duct Tape. You may also want some clear tape for taping the knot grid to the mandrel (see step #3).
5. Pins. In some of the pictures you'll see that I'm using threaded pins that I got from knottool.com: http://knottool.com/th_tubes.html. T-pins are actually a bit easier to use and more accessible!