1. Pour the prepared jam into a glass bottle and tighten the cap.

2. Put the filled jam bottle into a pot, pour water until it reaches the middle of the bottle, and cook for 20 minutes.

3. After taking out the jam bottle, turn it upside down and cool it for 24 hours, so that the air in the bottle will naturally become a vacuum state.

4. If the lid of the jam bottle dents downward, it proves that the vacuum is ready. If it can still move up and down when you press the lid, you can tighten the lid and repeat the previous step.