Cascade control isn't magic. It's just that instead of using the manipulated variable of the master loop to control a real-world final control element you use it as the setpoint of the slave loop. In your case, the output of the level control loop will be the setpoint of the flow control loop.
You won't get many guys on this forum to do much of your classwork for you. We want you to learn, and you do that by trying yourself, and if you fail trying again.