There are many examples of wavy borders to be found on the net and it's most likely caused by sewing a border onto the edges just by matching the border length to the edge. That's almost guaranteed to make a border that isn't flat. Most instructions I see say to measure across the center of the quilt to get the length of the borders. I don't know about anyone else but when I'm working by myself it's awfully hard to measure across 5 or 6 feet and to make sure that my tape is exactly on one end while I read the measurement on the other end.
After squaring up one side of the border strip I pin it to the center of the quilt that I have carefully laid out, making sure it is square. Pin it to the quilt, matching the edges. Pinning straight down into the carpet helps to keep everything in place while you do the other side if you can.
Very carefully lay the border across the center of the quilt, not pulling or stretching, and gently smooth it. Pin the other side just back from the edge. Either slide a mat under the edge and rotary cut it using or mark the line square with a ruler and use scissors.
Unpin the border and use it to make another one the same length. I fold the border in half and pinch a small crease at the center mark. Do this on the quilt too. Match up the ends and the center and pin the border to the quilt. There will likely be a little fullness to ease in so don't panic if you find that the border is shorter that the quilt edge. Pin them even on the edges and the middle and ease the quilt edge to the same size as the border.
Do the same with the other two sides after sewing on one set of borders and ironing them well.