Since then, I've made several more and learned that the key to successful sewing it together is to make sure you have an even number of running stitches on each side and the same number of stitches on each side. This ensures that the piece can accurately be divided in half - helpful when sewing the last half of stitches on one side to the first half of stitches on the other.
|Photo - Nordic Needle|
I took a look at my running stitches and noted I had an odd number. The fix was easy - just make more running stitches one thread inside the original border. This creates an even number of stitches. That done, the pieces sewed together really easily.
When I did a search for the Nordic Needle photo above, I found all kinds of intriguing images of biscornu patterns. They bear closer inspection...