To use the thermometer, make sure it is clean and dry. Apply the thermometer to the recommended area: armpit, mouth or rectum, depending on the manufacturer's instructions and the baby's age. If measuring in the armpit, place the tip of the thermometer in the baby's armpit and keep the arm pressed against the body. When measuring oral, ask the baby to keep their mouth closed. For rectal measurements, which may be more accurate for young babies, follow the manufacturer's guidelines carefully.