Sign dark by taking both your hands open, with forearms held up and open palms facing you, fingers directed upward. Then fan your two open hands downward across your body, crossing each other. The sign looks like you are bringing down shades to block the sun.
You can use the dark sign in a variety of real-life situations, like when you go into a room without the light on, or when you go outside at night. It is often helpful to teach dark together with its opposite light. You can make a game out of switching the light switch off and on, and signing dark and light.
The dark flash card works well with the light flash card to help your baby learn the meaning of the two words. Supplement the flash cards with lots of real-life examples of light and dark.