I do basically as dunster stated.
First I measure the length from the center of the quilt top to get my measurement. I sew the long sides first. Then I measure the width from the center of the quilt to get my measurement then I sew my widths. I do add a tad more for wiggle room. Better to have too much than not enough. Just trim the difference. If I want one finished 5 inch border I'll add 1/4 inch to my border strip and cut at 5 1/4.
Also, I find my centers of quilt and borders and attach with pins from the center working to the left then from the center working to the right. I was told doing it this way will keep you borders/quilt from being wavy.