@handle Diceware is for encryption key seeds imo.
Guess and check style passwords, like a debit card pin, don't actually need to have that much entropy in them. They just need to not follow obvious patterns and to be unique. And unfortunately, they also need to be flexible enough to adhere to whatever stupid draconian password policy will be forced on the user.
If I was going to teach someone one thing about passwords, I would probably teach them Password Manager + paper backup. Am I cheating? Is that two things? I don't know. If I had to cut it down to one thing I might choose paper.