Cart,
The example I was giving is if there is no intersection where the 2 airways cross, but Falconpilot is also correct. If the crossing airways have an intersection, just build your flight plan to the crossing intersection using the #J123 method and terminate at the crossing intersection. Then you enter the new airway from that crossing intersection, but more often ATC isn't that nice. More often you will get one jet route to join another at an undefined point in space. This is where the Pseudo-VOR comes in handy.
Like I said, the best way to learn a box is plug in power on the ground and just play. Also, there is a simulation disk the Honeywell put out for the GNS-XLS. I had it at one point, if I find it I'll burn you a copy and send it your way, but somebody else here may have it. Pretty good simulation that has all the functions of the XLS, but is interfaced with a Cessna CJ type cockpit. You can do everything with this sim that you can with the box.