Agree out loud how the team will behave — how you speak to each other, what happens when something goes wrong, what is not acceptable. Written down, agreed by everyone, visible.
Every team already operates on rules. Whether it is acceptable to be late, whether you can challenge the boss, what happens when somebody makes a mistake, whether people cover for each other. Nobody wrote them and everybody follows them.
The exercise makes them explicit and turns them from inherited habit into a deliberate choice. The team writes them together — imposed rules are just policy, and policy does not change behaviour.
An unwritten rule cannot be pointed at. When somebody breaks a written rule the conversation is about the rule, not about them personally — which makes it dramatically easier to have. That single shift is most of the value.
The test of a real rule is whether anyone would be uncomfortable if it were broken in front of a client. Anything softer than that is a preference.