Seriously, don't try this project. Do not click the green flag. It is definitely not mobile-compatible... Don't even think about the green flag. Think about... Microsoft Paint! Yeah! Go there instead.
Click to draw a line / move the pen anywhere on the board. The first button undoes the last move you make. (Let me know if the undo should undo the last DRAWN line instead of just the last move). The second button erases and redraws your picture, entirely from memory. Just a cool thing I thought i'd add in. The third button erases the board and the memory (start from scratch). The fourth button is a toggle between pen down and pen up.