Hold the diopter directly in front of your lens. Position the glass edge to align with a vertical or horizontal line in your composition to hide the focus transition.
Yes. Because it is handheld and 150mm in diameter, it is large enough to cover almost any professional cinema lens front element without needing specific threads.
The number indicates the strength of the close-focus effect. A +2 is the industry standard, providing a significant focus shift for most medium-to-wide focal lengths.