Machine piecing depends on sewing exact 1/4" seam allowances. Some machines have a presser foot that is the proper width, or a 1/4" foot is available.
To check the width of a machine’s presser foot, sew a sample seam with the raw fabric edges aligned with the right machine’s presser foot. Measure the seam allowance using graph paper with a 1/4" grid or a quilting ruler.
Another way to check your seam allowance is to cut three 1-1/2"-wide strips of contrasting fabric. Sew two of the strips together, using a 1/4" seam allowance. Join the third strip, again using a 1/4" seam allowance. Press the seam allowances away from the center strip. On the right side of the fabric, measure the width of the center strip. It should measure 1".
If your center strip does not measure 1" wide, you did not sew a 1/4" seam. Repeat until you get a 1"-wide center strip. Retest your seam guide from time to time.
If your quilt has many fabrics, use a neutral color, such as gray or beige, for both the top and bobbin threads throughout the quilt. Using two different threads, one on top and one in the bobbin, can help you to better match your thread color to your fabrics.