You need to measure your quilt through the middle, then cut your borders to that measurement. Fold the quilt and border in half and in quarters, pin halves to halves, quarters to quarters, ends to ends, and then sew.
When you simply sew a border on to a quilt without any measuring, there usually is some stretching so that excess border is sewn on to the quilt. This is what creates the ruffly border. If you were to measure your quilt edge and your border's outside edge, you would find that the border is longer than the quilt. This is even more likely to happen if you sew with the border underneath the quilt, because the feed dogs will feed extra fabric with every stitch. Sewing with the border on top and using a walking foot can help with this, but really there is no substitute for measuring the quilt through the middle and cutting the border to that length, then pinning before sewing.