I make my snowball squares as above, sewing just to the side of the line and only cutting off the center fabric and leaving the original square in tact. Leaving the original square in tact is probably the most important tip. One thing that has helped me too is to use Aurifil 50 thread which is a finer thread and a 70 or 80 needle which is also finer. The stitching now takes up less space so my folds take up less as well. Starching also helps get a crisp fold and less stretching so your finished size is more accurate.