Press space to move to the next slide. There are three slides for the first wave, so once you're done with the third slide, just press the up arrow to go to the example. The example has two slides. Once again, press space to move to the next slide. Then, it will be taken to the get ready screen. When everyone is ready to start the timer, just press space one last time.
Note: This is for a contest that I am running. Music Credits: Racer X Scratch library