"Rain, rain, come this way, do not wait another day."

Repeat the phrase about 8 times or however many you wish.  (8 is my favorite number so I use that.) As you say this you should be holding the lava rock in your right hand spinning your hand around in circles towards the sky.  After you're done, the first hint that it worked would be the wind picking up strength. After that you just have to see what happens.  Most of the rain storms that I've made have only lasted for about 5-10 minutes.