I measure down the middle of the quilt top lengthwise. That will be the length of the side borders. Cut the borders. Find and mark the halfway point, use a pin or mark with a washable pencil. Fold and find the middle points on the sides of the quilt top, mark with a pin or pencil. Match the middle of the borders to the middle marks of the quilt top. Pin the middles together. Then pin the entire border on, starting at the ends, so that there is no material overhanging, the ends and middle match up perfectly. By putting one pin in the middle and starting the rest of the pinning at the ends, you can account for any slack and avoid the "waves".
Once the sides are on, I press the border out and then repeat the process, measuring across the middle of the quilt top width-wise, to give me the length for the top and bottom borders.
Hope this makes sense!