Hold the gadget flat against the wall where there’s likely no stud underneath then press the button to turn it on. This is to calibrate the gadget. Alert signals will turn on and off.
Slide the unit horizontally and slowly while pressing the button. Once the light flashes or the sound went off, that indicates the gadget has found what you’re looking for based on your setting. It could be a stud, a pipe or AC wiring. 
Mark the spot with a pencil where the alert signal went off. If your stud finder is a multi-functional type and you’ve set it on finding metal, the signal or light flashes will indicate it found a metal underneath like nails, screws, or a metal pipe. Setting it on the stud, the alert signal will turn on when it finds a solid object underneath.