You really have 2 choices:
1. Purchase enough fabric so you can cut the border width for each side to the length (in this case you need 66" (or a little more to allow for unevenness in the cut).
2. You can cut the fabric cross-width and sew the two pieces together AFTER trimming the selvage. (I always take off the selvage because it is a tighter weave) I generally do not like to sew the border pieces on the diagonal. I sew straight acress and unless it is important to match a pattern, I don't even care where I cut them. If you piece your border it is nice to plan where the seam will be on the top. Some experts suggest not to plan it where the quilt will be folded. I'm usually not that worried about that.