Final answer:
The first step in a shampoo procedure is to test the temperature and pressure of the water to ensure client comfort and safety.
Step-by-step explanation:
The first step performed during a shampoo procedure is c) testing the temperature and pressure of the water. Ensuring that the water is at a comfortable and suitable temperature and pressure before starting the procedure is crucial for the comfort and safety of the client. This step precedes others like brushing the hair, applying shampoo, or positioning the cape.