Touch the greeb flag to start. Move your finger to move the paddle. Touch the ball with the paddle to make it bounce. Don't let the ball touch the blue line. Don't move the mouse in the white surrounding of the game. After setting the ball's speed, click the green flag again. Then press space.
Thanks to my teacher who helped me do this. Also play my two-player ping pong at: https://scratch.mit.edu/projects/370692098