The exact way to use the handheld meter depends on what you are actually doing. If you are lighting up a space for the shoot, you will measure the ambient light. So, you can place the meter at different points in the set to check if the area is evenly lit. On the other hand, if you are shooting a subject, you will need to measure how much light is hitting the subject directly. To measure that, you will need to place the meter right next to the subject and take the reading. Another method to use is called reflective spot metering. In this method, you are actually measuring how much light is actually entering the camera sensor.