Only bad advice was that it was necessary to prewash any fabrics.
I almost always prewash as a personal preference, not that it is necessary.
I piece my borders on the diagonal and cut lengthwise, and measure the quilt top first before cutting border to length. I pin from the center then the outside corner then inbetween. Never have wavy borders.
I teach quilting 2 to 3 times a week, I tell my students there are no quilt police. Explain why I it things "my" way, but there are other ways and find what works for you.