Drag the little 1 and 2 markers around with the mouse, use the 'Z' key to change the color, the 'X' key to change the size, the 'C' key to clear, and the Space Bar to draw a box.
Shared from Scratch 2 Offline Editor I know it's a little glitchy right now, and it may take multiple tries to do something, but it will usually correct itself after a few tries.
