We have had several beautifully pieced quilt tops come in for quilting, with REALLY wavy borders. Unfortunately some of these can't be smoothed out and wrinkles of excess fabric are unavoidable. It breaks our hearts to see this, especially when a simple solutions would make this a non issue.
These large waves are caused by not measuring the border fabric before attaching but simply taking a large strip of fabric, sewing it on and cutting off the excess.
Here is just one of MANY tutorial online about how to sew on borders....
Best Borders!!All of the rules, tips and tricks to finish your quilt!