Zipper is a natural sign. Take both hands and pinch the thumb to the index finger on either hand. Your non-dominant hand stays put at the imagined location of the zipper, while the dominant hand makes the motion of unzipping and zipping from the "base" where the non-dominant hand is positioned.
Make the sign for zipper wherever you want your baby to mimic the action, on his or her clothing or backpack, for example.
Teach your toddler the zipper sign when you want to indicate to them how to open and close a zipper. This is useful for toddlers who are developing autonomy in dressing up and preparing their backpacks.
Use the zipper flash card mainly with a toddler who is learning dressing skills. Consider using a toy that includes zippers and buttons.