Move the mouse to move the pen, and hold down the mouse to draw. To change direction, let go of the mouse, change direction, and start drawing again! To look at your masterpiece, hold L and the pen will disappear, leaving only your drawing to see. To lift the pen, hold space and use the arrow keys to move it around the drawing pad.
Thanks to @DuckBunny2. All credit to him. I thought it would be better if some features were slightly different, and fixed a few bugs, so I updated them. Check out the original here -- https://scratch.mit.edu/projects/43197796/ V2.1 - Fixed expand bug (the drawings would expand)