Take the black border off.
Starch well and press the black border. Sew a line of stay stitching down each side of black border. I use 2.0 stitch length and sew inside of a 1/4 inch seam. It won't show when you attach the borders.
After the stay stitching, press again. This second pressing flattens the fabric.
Remeasure the border length needed. Cut to exact measurement. Mark the border at half point. Mark the quilt border at the half point. Pin these two together. Mark and pin at the quarter points on each piece. Don't be stingy with the pin process. I pin about every 4-6 inches along the border. It's just a little work for a better result.
Best wishes and good luck for flat borders!