to Amybaby,
Precision starts with Washing, pressing, starching fabric. This process helps to shrink and straighten the grain of fabric.
Then get a large rotary cutting mat with the grid printed on it to help keep everything straight and a yard long cutting ruler with grid, oh, and a good rotary cutter. I worked at a sewing store for two years and had to "finish" a small log cabin quilt another employee had started for display, then left on "medical" leave and never came back. First I had to take the whold D--- thing apart because nothing fit right. NONE of her strips were the same size, so I had to recut most of them. Yeah, I finally got it done. We should have classes on fabric cutting for beginners.
NOTHING you sew will fit together properly unless it is cut correctly. Then sew STRAIGHT. Slow down. There is no prize for getting to the end first, and it has nothing to do with the price of your machine.
Paper piecing is a lot of work just to avoid sloppy cutting. Also take the time to press before sewing on the next strip.
By the way, there are some very nice "wonky cabin" quilts out there, but they are deliberately cut to go off square for the effect.
Hey, now we have a use for all the fractions and other math they made us learn way back in elementary school.