Several tips together here on how I cut long borders.
I first cut yardage from the length of grain, or piece segments from whatever I have, to a length a bit longer than I know I will need. I prepare two pieces, one for each side.
I cut two at once to ensure the quilt will be square.
Because I am pretty accurate with my sewing, I calculate what the border should measure. I add up the block measurements and sashings if included (this works with straight sets, not diagonal sets). I take this measure and divide by two. And I circle this final number.
I divide by two and get 53.5.
My mat is 36", so I take a ruler, extend left from the 0 by 17.5 inches.
Place a pin in the fold of each border.
Then place another ruler at the 36 inch mark.
I get the quilt, fold it in half, and lay it along the border. (The border might shrink a bit from the time I was smoothing--this is ok. Just make sure the left ruler remains in place.)
In this step, I fold the quilt at several places--the sides, middle, maybe one or two more. I watch to see that it measures close to what I want to cut it. I can always ease a bit.
This step ensures that my calculations are correct--I have several times discovered I was off by an inch or so, often by forgetting to calculate a first border correctly.
Also, I may find that the quilt top measures consistently larger or smaller than calculations predict--in which case, my sewing WASN'T as precise as I thought, and I know I need to cut my border a bit longer or shorter.
Once I am satisfied, I make the cut.
Pin the start and end, and let the quilt tell me where I need to do some easing, pinning well.
Soon, how to put on a border with no pins!