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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.