Your waves could be that you are stretching the fabric(s) because you are sewing on the bias.
The key for me is ... do not do it when I am the least bit tired!
What works for me ...
* Best Press the fabric well .... at least in the mitered zone.
* Leave the border pieces the full length/width.
* Mark the 45° line on the wrong side. and pin the pieces two together.
* Stitch.
* Press. Open. Press. Check!
* If all is square and flat ..... then trim the excess material.
* Repeat for the other four corners.