You can do this one of two ways. I personally prefer to use a washcloth, but I've done it with just my hands before. It's just as fun either way though, so pick one and try it out!

1. Get into shower. Put shower on.

2A. Hold washcloth to stream of water

2B. Put hands in a V shape with the point at your wrists. Tilt hands towards self slightly. Keep this shape and hold it to the stream. Adjust shape as you see fit.

3. You made a waterfall!