According to Blanchard et al, you must master three key skills to become a top trainer.
One should be able to assess your trainee's competences as well as their commitment to their tasks.
You should be able to switch between leadership styles from trainer to trainer and also when individual moves from one development level to next. (please refer to the diagram)
Finally it is crucial to have sufficient communication between you and the trainer selected. They need to understand the rationale behind the way you manage them