How do I handle overthrows if the ball comes to my end?

You don't need to signal overthrows. Runs and overthrows are counted by the scorers and signalled by the main umpire.

Your job at your end is to watch for a run out if the ball is thrown at your stumps, and to check the batters ground their bat or a foot behind the crease each time they turn. If a run wasn't completed properly ("one short"), tell the main umpire.