Using the fabric on the lenght of the grain ( rather than cut selvage to selvage or width) keeps the border from having stretch. Many post have been made here about wavy borders, and they primarily came from the same issue , using border fabric cut selvege to selvedge or width of fabric.
It is tough to cut those long pieces, But I first cut the selvage off the lenght , then cut on the floor moving my cutting mat under the fabric as I go. I use two rulers one for the cutting , one for keeping the width . I line the cutting ruler up against the width ruler. You can double fold the fabric to make it a bit easier.