As you demonstrate, point out the important aspects so that the C.A. understands why each part is important. Nothing is worse than having to experiment with a new skill on a patient. Always give the employee an opportunity to practice with you first. Give the employee feedback on what was done correctly. Too often managers only correct mistakes.
Positive feedback is much more important in training new skills.
Once you have explained what was done appropriately, give feedback on what needs to be improved. Supervise the skill in a real-life setting. This last step gives the employee an opportunity to ask questions if necessary. After you watch the skill, give feedback. Be sure to point out the strengths before you point out areas that need improvement.