You didn't tell us what kind of tools you have. I will assume that you have a table saw. If you also have a dado setup then it is "easy" to do a box joint. Make a wooden fixture that will attach to your Miter gauge. You need to decide the width of the box joint and drill a hole in the wooden fence for a dowel the same size and the box cut. Then you make a cut, insert the dowel place the cut over the dowel and make the next cut repeat to the end of the board. use the first board to mark the other board and make the cuts the same way.
Hope that helps. There are some good books that show this method and I have seen Norm use it on one of the older shows. You can buy his old shows on Video.
Here is a plan to make a great fixture. It's called "Lynn's Box Joint Jig" and works really well. It allows you to cut perfect box joints for less than $10 worth of parts and WITHOUT a dado blade. It also doesn't suffer from the cumulative errors that the dado and pin jig type jigs do.
Amazing how accurate a low tech chop built fixture can be, eh? It's all from the leadscrew. There really isn't any cumulative error and you can micro adjust. Neither of these features are possible on the dado & pin type jigs. The only thing that I've found to do a better job is my $400 Incra TS-III and the router. - 40x the price for about a 10% quality improvement - and harder to dynamically adjust to boot!