I would sew it in just like putting the top on a baby blocks you don't have to worry about the angle sew the white square to one side being sure to keep your fabric straight stop with needle down in center of your seam between lavenders ... turn and sew down the other side if you left quarter inch at your seam it will turn easy if not no problem just go ahead and stitch it them maybe have to remove a couple stitches in your lav pieces you will know when you iron it but you can sew either way