Hi - many times the wavy-ness is due to the way you put the border on the quilt. Make sure you cut the length to the length and width of the quilt. Add the appropriate amt for mitered cornors if that's the case. Find the center of the strip and place on the center of the quilt edge. Pin your border in place. If you just cut the border and start at both edges, your border will be wavy, instead of flat!
Terri