The sign for new looks like you are scooping or shoveling up a new plant from the ground. Your non-dominant hand is flat, palm facing up, to signify the ground. Your dominant hand is a curved open palm that makes a scooping motion into the "ground" and ends facing up and above a short distance from the ground.
As your infant gets older, you can introduce the adjective new. For example, your baby might start distinguishing two different blankets by talking about his new blanket and his old blanket. We often use new when helping our babies make observations about gifts, or when shopping for something new.
New is a hard sign to teach from the flash cards. A better way to teach the sign is to use two similar old and new objects, and teach the two signs by contrast to each other.