I would measure each point to the edge of the fabric. Then I would use the measurement that is the smallest (say your measurements are anywhere from 1" down to 1/3".) Then I would make a mark straight out from the point to 1/3" out. Then cut from point to point and then your octagon would be "square". Once your Octagon is square, you can measure from the center out to the size of square you want the pillow to be. Make sure each side is the same width from the edge of the fabric Hope this doesn't confuse you more.
Last edited by QandE2010; 11-04-2011 at 10:37 AM.