When you buy a small bottle of yogurt in the supermarket, it must be fermented from local milk, not from yogurt or lactic acid bacteria. Buy 4 packs of fresh milk, cook it, and let it cool. Be sure to wait until it is completely cool. Pour the small bottle of yogurt into the cold milk, stir well, seal it (it is recommended to use Tupperware or lock lock's fresh-keeping box), and put it in a hot place for fermentation. If it is not hot enough, wrap it in a small quilt or something like mash. Almost a day, it will be fine.