Step 2: Prepare Cardboard
The easiest place to find a large piece of contiguous cardboard of the appropriate size is from a large box. You want to cut out a rectangular piece of cardboard about 19 and a half inches by 15 and a half inches. If you don't have a box that large around your house, you can try looking for one in a grocery store or Walmart; they're usually happy to get rid of boxes. I used a box from a 20" floor fan.
It may be useful to first cut out a piece of cardboard slightly larger than needed (say 20" by 20"), then measure with a ruler or tape measure and cut to make the cardboard the correct size.