After you press start, the spinner will spin around to point at a random point. Use your mouse to block the ball from going to the edge. It's pretty much just pong with yourself. Every time you hit the ball your paddle gets smaller. When an icon appears in the middle try to hit it to make your paddle size go back up to 100%. The ball gets faster as time goes on. When you die, after your score appears, a restart button will appear and when you click it, you will restart.
I got the basic bouncing movement from FUNUT (Professor Blue on youtube). Go check out his scratch profile and YT channel.