Just click the flag once and the game should begin or maybe you wont have to idk. The paddle follows you´re mouse pointer so i think full screen would be best for this. If the sun hits the red line the game stops and you have to click the green flag again. It should still keep the score tho because i don´t know how to delete it if you hit the line.
I kinda just messed around with stuff until a friend came and helped me understand what to do xD