Assure the employee of your confidence in her success, and of your readiness to help her achieve it. Once you've handled the immediate challenge of returning unsatisfactory work, take time to consider whether there were any steps you could have taken to help the employee succeed the first time around. The next time the employee takes on a task like the one that gave him trouble, make sure the same problem doesn't crop up again. Reinforce the worker's understanding of his goals, and make sure he has the time and tools to achieve them. And when he succeeds, be sure to praise his progress.
Successful results happen when the employee-supervisor partnership stays on track.