Move your hand onto the mouse, and drag it over until the little cursor hovers over the mutated green flag, and press. Or, do the same with a touchpad. Or or, simply press the mutated green flag with your finger if you have the luxury of touch screen.
I had to make all this on an XBOX as I don't have my PC at the moment, so I kept getting memory limit exceeded warnings because consoles aren't really made for browsers, and more specifically using Scratch.