From Vintage Clothing Construction Helps, advice for beginning sewers which begins with the recommendation that one practice learning to stitch straight on the sewing machine by using an unthreaded machine on paper. Then, once proficient:
Before stitching on garment, check the tension on the machine and the length of the stitches. In the correct stitch, the upper and lower thread should lock at the center fabric (figure 2).Here's figure 2:
This vintage booklet was by Margaret Fobes, Alice Linn, and Ethel Gorham, Extension 4-H Bulletin #13, February 1944; a 4-H Club booklet on sewing clothes put out by the University of Minnesota, Agricultural Extension Service, U.S. Department of Agriculture, University Farm, St. Paul 8, Minnesota.