Take your time and give extra attention to this step. You'll be much happier with the results.  Otherwise your border can be a real trouble maker!  The time taken to do this extra measuring and pinning will pay off with a smoother border.  Borders that are not smooth can appear puckered once quilted.  In extreme cases a small dart may be needed to absorb extra fullness.

Lay your quilt top out flat, smooth and take three measurements from top to bottom (1.) one at a few inches from the left edge, (2.) one near the middle, and (3.) one at a few inches from the right edge. 

Average your measurements if necessary.

Cut 2 strips.  Divide into quarters and pin onto the quilt top before sewing.  Some easing may be necessary. 

Now you're ready to cut the remaining two borders.  Take three more measurements this time from left edge to right edge (1.) one at a few inches from the top edge, (2.) one near the middle, and (3.) one at a few inches from the bottom edge. 

Average your measurements if necessary.

Cut 2 strips.  Divide into quarters and pin onto the quilt top before sewing.