The four patch units have setting triangles in the corners. The units are set on point and the size of the squares 1 3/8 wide. If not made precisely, the squares will not fit properly and it will be obvious in the quilt.
Cutting the Squares The first step is to use a sharp blade in your rotary cutter. Otherwise, a few threads will be skipped in the cutting and the fabric will move under your ruler.
Last year I bought a rotary cutter sharpener at the 2010 Houston Market from The Grace Company. I believe it has paid for itself by lengthening the life of my blade. The Grace Company has two sharpeners for sale, but I have only tried one of them so cannot compare them to each other.
Run the blade back and forth about 20 times, turn the cutter around and repeat.