My practice with borders is to "Sew as You Go". I have never had a wavy border or such problem! With the border fabric cut a few inches long, I lay it gently on the quilt. No stretch to fit or need to push the ends in to fit. I pin at about 4" from the start and about 4" along from there. When I sew I remove each pin as I get to them and put it another 4" along. Never do I try to fit the whole strip to the quilt at once! Try this on a practice piece!