This is so important! Measure across the quilt center in three places - on each side and through the middle. Are
they the same? Probably not, but hopefully they are very close.
If your measurements show a large discrepancy, square up the top before going any further! Re-measure to check.
Take the average of the three measurements, and cut the border fabric to that measurement, plus whatever you
need for the corners. Lay the top on a flat surface, face up, and place the border in place face down on the top.
Smooth it out gently and check that the ends meet. Pin every few inches. Sew, removing pins as you come to them.
Press seam toward border. If you notice flaring, stop and fix it!
Many times the borders must be pieced to have enough fabric. As a longarmer, I have found that diagonal seams in
the border are a major source of fullness in the border, and a problem in the quilting of the top. That diagonal
seam is on the bias, and subject to stretching, and more difficult to get precisely sewn. For my own needs, I
prefer a straight seam when I must piece the border fabric.
Never use the selvage when piecing any part of your quilt top - not even in the seam allowance!