Understanding the eating rules in Japanese culture will help you integrate more easily, and applying these rules will also help you limit unpleasant looks from Japanese when interacting.

1. Don't forget to say “itadakimasu “ before every meal.

    “itadakimasu” means the person thanked the animals and plants that gave their lives, so we could eat. As well as thanking all the people involved in getting us the food.

2. Also don’t forget to say “gochisōsama-deshita “ after every meal.

    “gochisōsama-deshita “ means “It was a great deal of preparing the meal” or “Thank you for the meal”.

3. Don’t stab food with your chopsticks.

     Use chopsticks to pick up food slowly and gently.

4. Never rest your chopsticks on the bowl.

     In Vietnam this is normal but in Japan it is not.

     Using your bowl as a resting place for your chopsticks rest is a breach of etiquette. If no chopstick rest is available, use the wrapper the chopsticks came in to make your own.

5. Do not use the chopsticks you are using to pick up food for others.

     If you want to pick up food for others, it's best to get a new pair of chopsticks to pick up food for others.

6. Do not point with your chopsticks or wave them in the air.

     Pointing with chopsticks like pointing a finger at someone. Or waving chopsticks in the air while talking is also rude.

     Make sure to rest them before you start telling a story with lots of hand gestures.

7. Eat all the food you put in the bowl whether you like it or not.

     Don't leave it to waste. If you can't eat much, pay attention to taking a moderate amount of food from the beginning.

8. Remember to return bowls, plates, chopsticks, and bowls to how they were at the start of the meal once you’re done.

     This is especially important in Japan.

9. Be conscious of how much soy sauce you need to pour appropriate amount.

     In the small bowl provided, pour only as much soy sauce as your meal requires. Over serving soy sauce is considered bad manners.

     In addition, don't pour soy sauce directly onto rice, sushi or sashimi.

Above are just a few typical notes that I concluded after a period of interacting with Japanese people and their culture.
And of course, there are still other interesting notes you should learn more!