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I should have gone another stitch or two here, but you get the idea. A perfect 45* stitch is best here, but it is somewhat forgiving.
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This pic is upside down, but you see that when you sew that 45*, it gives you a guide for your fold.
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Fold your binding at the 45* angle, then back down the edge of the quilt. Go all the way around the quilt in the same way, stopping about 18" from where you started.
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You now have a gap of 18" or so, and 2 tails.
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Make sure you have enough tails to start trimming before you cut anything! When you have confirmed you do, trim one tail straight near the center of the gap. Make sure your quilt is flat and smooth on your surface. Lay the other tail over the first one and measure an overlap of 2.5" plus 3 or 4 threads. Cut the second tail.
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Fold the right tail upward at 45*.
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Leaving the bottom half of the left tail in place, open the left tail and lay the top corner onto the top corner of the right tail.
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Pin the tails together, keeping the corners lined up. You will be sewing from the upper left corner to the lower right corner of the join.