Use the arrow keys to draw. A and D to change colors. W and S to change sizes (W makes bigger and S makes smaller). Space for new backdrops. Q to clear all off it. Click on the stamp (it starts as "Awesome") to use it. R to stamp it. E to change the costume of it. F to stop the stamp.
You might need to double click the flag to get it to work. If changing the color, size or backdrop doesn't work, just try one more time.
