The 2 biggest problems I see with new quilts coming in are 1) the backing is too small, and 2) the borders have fullness & don't lay flat.
The backing needs to be 3" bigger than the quilt top on all sides. So, if your quilt top is 70"x90", then your backing needs to be at least 76"x96". This way, if I need to trim a side to get a square back, then there will be enough fabric to work with.
The wavy border problem can be fixed by properly sewing them on your quilt. If you just sew on the border, and then trim off any excess, it's very likely you will end up with fullness in the border. If you measure the middle of the quilt, before applying the border, cut the border to that length, and then pin in place & sew on, you will have a much better chance of ending up with a flat & square quilt.
Follow this link to my website for a very short illustration of how to apply borders: