Put it at tortoise level directly under the UV source, point the sensor directly at the bulb, hold the button down and move it around while looking at your reading. I usually dig a hole in the substrate and rest the bottom of the meter directly on the bottom of the enclosure.I bought the much insisted upon solometer but now I realize I have no idea how to use it in the care of my 4 month old Hermann Tortoise. Could someone please tell me what to do with this expensive gadget?