To sign naked, touch your middle fingers to you chest, and swipe down along your body, as if you were brushing off your clothes.
As an extra part to the naked sign, hold out your non-dominant hand flat, with palm facing down. Then with the middle finger of your dominant hand, touch the back of your non-dominant hand, starting from the wrist area, and outward in the direction of your spread-out, non-dominant fingers.
Most infants seem to enjoy romping around naked, and so when they are gleefully crawling around the house without their clothes on, sign naked.
You can teach the naked flash card together with the clothes flash card. The contrast helps make the understanding of each word and sign more clear.