Put the appropriate knowledge in their heads and put appropriate knowledge in their hands. Well-designed manuals and job aids are far more effective than memorized facts. New trainers should learn how to create or select good manuals . They should learn to create and use job aids. The simple answer is to teach trainees how to perform the tasks required to do a job.
Levels can range from simply knowing the task exists to being able to teach it to others.
Trainers also need to decide whether each task should go into the trainee's head or hands and the right way to teach a given task. It takes training and experience to truly understand the best teaching methods. In a perfect workplace, trainers would learn skills, practice techniques, and prove their competence before affecting other employees. In the real workplace, they can at least experiment with various teaching methods. Once is seldom enough.