Snorkel mask fitting guide. Now, with the strap still over the top (not over your head yet), put the mask to your face, press slightly, inhale a small amount through your nose to suction it to your face, and release your hands. The mask should stay on your face without continuing to inhale. You should be able to move around a bit without it falling off. Try moving your face muscles a bit. Does that break the seal? Try smiling. That will often break a seal on even a good fitting mask. But see what it does. (Sometimes we try to get each other to laugh when snorkeling to cause each other's face mask to flood).