This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
First of all, you'll need to make sure your panel will use the entire JApplet size. Since JApplet uses a BorderLayout by default, that's not a problem. Just add the panel to the center of the JApplet (using BorderLayout.CENTER).