click the green flag to start. you can change the the sliders to make different patterns. press space to erase everything click the plus sign in the top right corner to increase the pen size. The large readout next to the plus is the pen size. for an anti bug mode, hit the heart and star at the bottom of the page ;)
I got the idea from a DK scratch coding book and did a couple of mods to it. Please report any bugs or glitches. For no bugs or glitches, smash the heart and star buttons :). But seriously, when I hit the heart and star buttons the program ran more smoothly.