Quilt preparation is important to the outcome. Quilts should be pressed with attention to the seams and threads trimmed.
Seams on pieced backing should run horizontal for best results. Backing and Batting should measure a minimum of 4-6 inches wider on all 4 sides and not more than 8 inches.
To avoid wavy borders, take a measurement horizontally at top, middle and bottom of quilt. Average these three measurements and cut top and bottom border to that measurement. Work in fullness if necessary. Turn quilt and repeat the measurements (including top and bottom border) for length of quilt, measure and cut border for length. Work in fullness if necessary. Repeat for each additional border. This will keep your quilt square.
It is preferred that minky backing is not pieced.