Fabric can stretch. It can happen from handling the quilt top, from ironing, or because there are a lot of bias seams. No matter how carefully we handle a quilt top, there is always a chance that the sides are stretched out of shape. If they are a bit longer than they should be and if you then just add the borders they could end up wavy. A little bit of extra care is necessary to avoid this.
Measure the quilt top in 3 places, top, bottom and center. Add the 3 measurements together and then divide by 3 to get the average. If you get a really weird number, round it off to the next 1/8”. Cut two borders to this exact size. Before you add the borders to the quilt, find the centers on the quilt top and mark with a pin. Then, find the half way spots between the center and the top and the center and the bottom and mark those also with pins. Repeat this for the two borders, but only on the sides that will be attached to the quilt. Pin the borders to the quilt top matching the pins. Ease in any fullness. Add more pins, if necessary. Stitch in place.
Press the borders and repeat the same procedure for the remaining 2 borders, but don’t forget to add the widths of the borders to the width of the quilt top.