I just made little wraps for my daughters to wear over their flower girl dresses (same basic thing, a large rectangle w/ border. I used the following link:
Press the corner after you fold it, before you sew & it helps it lay flat. The only thing I had trouble with was how to connect the two ends of the trim to end it. I tried to make a seam but that didn't work out too well. Maybe someone else can help you with that.
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