Thank you Andir great handout, love it!!!

Thank you, I'm glad you found it helpful!

This happened to me too.... when I cut the border on the bias (did not want pieced border). I have learned to starch heavily (in addition to the measurement advice given by previous comments) and that has helped tremendously.

I agree that starching before cutting borders helps a lot too. Starch stabilizes the border fabric so it is less likely to stretch during the sewing process.

Pieced borders are another way to help with the stretched border problem. I like the look on my scrap quilts and it really helps with the borders if they are not just in one long section.

I agree with everything said, especially the link to the hand out. I would add one other dreadful thing we all try to avoid. PIN THAT BORDER TO QUILT fairly closely dividing fabric of border and quilt in half and machined EACH time. Taking the time to pin and ease can tame a lot (not all) wavy problems.

A few years ago I took a beginner class and that teacher also said to measure your quilt as mentioned above and then she always makes her borders 1/4" smaller than the measurement. I can't imagine 1/4" would make that much difference but that was how they teach.