The unmistakable "ribb-itt" sound comes from scraping the stick over the ridges on froggy's back. You can also tap the stick on his head for a wood block sound. The larger the frog, the lower the tone, so why not adopt the whole family?
Hold him in the palm of your hand, so he faces the same direction you do. To make him chirp, rub the wooden plectrum over the ridges on his back. It's important that you rub from tail to head. If you want to make him "ribb-itt", rub the plectrum from tail to head and then back down a couple ridges; all in one fluid motion. Now you can change the tone of the "ribb-itt" by covering and uncovering the holes at each side of his smile. Hold him nice, don't let him hop away!