That’s probably the easiest part. You hard boil eggs and then let them sit in a mix of mirin, soy sauce, and sugar. Some recipes add more ingredients to the marinade, but that’s the basic version. They can marinate for between 3 days to a week.
6.5 to 7 minutes depending on the size of your eggs. For flavouring, it's a general stock of ginger, soy, mirin, sugar which I recommend looking up and trying various blends for what you like best