To sign frog, make your hand into a fist and put it under your chin. Taking both your middle finger and index finger together, flick them outward off your thumb. The motion of your fingers is like the legs flicking out so the frog can leap from pad to pad.
Animal signs like frog are a great way of making a children's book more interactive. When there is an animal character, point out the picture and do the sign.
We teach the sign for frog contextually when we visit the local pond. We also have a cute rubber frog for baths.