Great job with the re-do. Glad you were able to fix them.
I stay-stitch using a 2.0 stitch length along each quilt edge and each border edge. Give both another press. Then mark and pin as described by bearisgray.
Stitch both together. The extra stay-stitching prevents the fabrics from stretching as they go through the feed dogs and presser foot. It takes less than 5 more minutes to stay-stitch.