Where are the waves happening? On the edges or the in the miter corners?
Are you cutting the borders cross grain or with length grain? How are you measuring and determining the border length? The methods can all factor in and result in a wave.
I had a project help class yesterday at the LQS. The instructor gave me a couple of tips. Starch the fabric and press before cutting. Then cut on the length of grain. Pin the borders to the quilt starting at the center point and quarter points. She also mentioned for my project to use a walking foot this time as one of the fabrics has a little stretch.