Use the mouse and press the button to draw. But just so you know, it's not gonna stay in place. Also, you can press [C] to clear the screen.
In case you don't get it, this is just for fun, and not meant to be a legit drawing program. However, I suppose it could be a fun tool you might find in Mario Paint or something. I was thinking of adding a save feature and the ability to change the pen size/color, but that would be too much work if only a few people are ever gonna see this. Also, the tags in the title are so this project actually gets seen. I'll remove them if the Scratch Team fixes it.