Old Youtube tutorial: Quick Tip - How to use Smart Masks
Creating masks in Corona Renderer is an easy process. You can create monochromatic as well as RGB masks simply by specifying Object ID, Material ID or by directly picking objects in the scene. You can combine different selections as union or intersection.
To add a masking element, go to: Render Setup > Render Elements, click "Add...", find "CMasking_Mask" and press OK.