Have you started a habit of ironing it so that either one, or both sides are stretching out of shape? I would do the 'usual' of measuring across the middle and cutting the piece to that length. Then pin it to the quilt at both ends and make sure the rest of it is eased in either by pinning first, or easing in as you sew. All the best with a solution.

I don't know if this helps or not to what you are talking about as I am new but I always start sewing my boarders on in the middle to the end flip repeat again from that middle to the next end. I also,don't know if it is right but I cut my boarders a little, maybe 1/2 to 3/4 of an inch longer, trim when done.

I cut the border on the length of grain which prevents stretching.

I fold the border in half and mark with a pin and then mark the quarter lengths. i do the same with the quilt top. Then I match pins and make sure the fabric is pinned together between the half and quarter marks.

I always cut my borders lengthwise rather than across the width and sew them on using a walking foot; no problems so far.

I cut my borders from the length of the fabric and use a walking foot when I sew them on. I pin a lot too.
If your machine is a Pfaff with IDT, you have a built in walking foot.
Sharon W.

When you sew that first border on sew with the quilt top next to the feed dogs.

Measure quilt as you have mentioned from center top and center side. Add a 1-2 to 1 inch to measurement. Cut borders on straight of grain lengthwise.
If you are using multiple borders construct them as one unit before attaching to body of quilt.
Find the center of the quilt side and the center of the borders. Match. When sewing to quilt start in the center and sew out to end turn and sew to other end. You can then square the ends to the quilt.
If you are mitering be sure and stop 1/4 inch from end.
This method eliminates stretching borders to fit quilt which is what causes wavy borders.
Happy quilting Severna

I quilted for about 25 yrs and always had wavy borders. Then I learned about cutting borders length wise and have had no problems since. Talk about a slow learner!LOL Recently I won a blue ribbon on a king size quilt and one of the feedbacks I got was about how perfectly straight it hung.

Are you using fabric straight of grain or cutting across the fabric? The "across" grain will stretch but the "length" of fabric will not stretch. Jan

I always measure through the middle of the quilt, cut both border pieces to that size. I put pins in the middle, divide each half in the middle, do the same on the quilt, pin together and hold quilt and border in between the pins and sew. I never have waves. I always cut my borders WOF.