Place the end of the tape measure at the corner of the exterior stucco wall.
Pull the tape measure along the surface of the wall and stop when you have reached 16 inches. The studs are typically placed 16 inches apart on an exterior wall. So there will be a stud approximately every 16 inches along the wall. Mark the spot with a pencil.
Place the stud finder on the mark on the wall that you made with the pencil. If the stud finder indicates that the stud is present, you can be sure that your mark is accurate. You may have to move the stud finder a few inches to the left or right of your mark to compensate for the width of the stud.