The leaf sign looks a lot like a leaf hanging from a branch, waving in a gentle breeze. Your weak hand acts like the branch. You point the index finger on your weak hand and make your other fingers into a fist.
Your strong hand is the leaf. Make an open hand, with palm down. Then place the base of your palm onto the index finger of your weak hand. Move your strong hand up and down a little to complete the sign.
As you take a walk around the garden or the park, let your baby bury herself in the fallen leaves. As you both pick up and take a closer look at each leaf, make the sign.
The flash cards are great, but where possible I always prefer to use the real thing. So go outside and show your baby a leaf on a tree – pick the leaf off and let her hold it, feel it, smell it, and most babies are probably going to taste it as well! The real leaf tends to get your baby much more excited and engaged that the flash cards ever can. Then once your baby has learned on the real leaf – you can use the flash card and all kinds of different real leaves to help her understand that the word applies to the more general concept of a leaf.