Regularly check the candidate’s work for accuracy together with him.
Review your candidate’s work with him and see if you can make him think of a way to improve that work next time.
Give your candidate ample time to finish his work so he does not feel forced to finish it in a hurry.
Ask the candidate to keep a detailed diary of what he did over the last week. Discuss the results. What did he spend too little or too much time on? Is there a pattern? Which distractions can be eradicated and how?
Investigate with the candidate what he can learn in terms of planning and organizing from his own work and, when appropriate, that of others.