This advice is late, but....
when I was a new quilter, I believe it was someone on this site that advised me to do the following:
When you are measuring for the border, measure thru the middle of the short side of quilt. Use that measurement to know how long to make the border for the shorter edge. Do long side the same way,being sure to add the width of the shorter border onto each end of the length from the shorter side. This method has worked for me to not have wavy borders.
If this is as clear as mud, I would look this up on line to perhaps get better instructions and pictures!