I have discovered that when I put together octagon shapes there is a trick to evenly distribute the folded over fabric in the ornament sandwich. If I fold the corners first on both the stitched piece and the backing piece then I will have many layers of fabric at the tops and sides, and only two layers at the corners. It looks something like this:
To avoid this I fold the stitched piece and the backing piece in a different order. For the stitched piece I fold the sides, top and bottom before I fold the corners. (As illustrated with with red paper) For the backing piece I fold the corners before I fold the top, bottom and sides. (As illustrated with yellow paper.)
Then the stitched piece and the backing piece fit together nicely and the layers of fabric are more evenly distributed.