To use a Quick Mask, make any selection using any selecting tool on any part of the face. See images below to see how to click on the Quick Mask tool. After selecting the Quick Mask tool, select the Paint Brush tool and "paint" your selection. Alternating between black and white as the foreground color to paint in or paint out the selection. Clicking back on the Quick Mask tool will alternate between the clear area looking like a mask and looking like a selection.