I saw Eleanor Burns take some fusible and place it on the right edge on the top, fold it under like a hem, then match it up with the other fabric, then iron/press it. Then she sewed the seam. I'm pretty sure she has done this in several of her tutorials. Have you seen that fusible strip that's only 1/4" thick - something like that I think is what she used. If you don't have any, I would just press the seam allowance under, then move the fabric around till it matches, then pin in place. Some people like elmer's glue and some a glue stick, to help hold it in place when you open it up to get to the seam.