What do I do when a runner leaves the base too early?

In Majors, when the pitcher is on the pitcher's plate with the ball and the catcher is in the catcher's box ready, runners may not leave their base until the pitch has reached the batter.

If a runner leaves early:

• Drop a flag (or otherwise signal) to show the violation.
• Don't kill the play. The ball isn't dead at the moment of the infraction, and the batter isn't out for it.
• At the end of the play, the umpire who dropped the flag applies the Rule 7.13 penalty. In the usual case the runner is sent back to the base they left; in some situations Rule 7.13(c) instead removes a runner, and the scorer records that removal.

Agree in your pregame conference who watches which runners, since the base umpire usually sees the early break.