The sign for poop is very evocative. You make both hands into fists, holding your non-dominant hand above your dominant. Then with your thumb from your dominant hand, extend it inside the fist of your non-dominant hand. Finally, pull your dominant hand and thumb down and away from your non-dominant hand.
Having your baby learn to start communicating to you about poop and pee is the first stage of potty training. To help them learn the signs and underlying concept, make the appropriate sign as you change their diaper, when they are on the potty, or if they have an accident.
Poop and pee are best taught exponentially, using the sign every time they use the bathroom or have an accident. You can add the flash card to help them learn to generalize and to help them learn to make the sign in anticipation of the event, rather than in the aftermath.