It's not a simple process, however Fanuc has it thoroughly documented.
I don't have RoboGuide open in front of me, but if you open the help menu (press F1 on your keyboard) you will find what you need.
are you familiar with machines and moving them? --> thats the way to go.
I believe there is an example workcell (inside RG folder) with aux axis.
And here inside forum there are severell post about this topic.
And there exist several YouTube tutorials about this task. But be warned; some of them are showing it the wrong way, don't remember which one I used at last, but some of them helped me.
I attached an example of how to create multiple rails in this thread.
