How to assign a switch in my Devo 7

I admit, I am a total newbie with quads. I am trying to install mission planner on my QR x350 pro. Mission planner says to assign select assign a function (number) to mission planner in my DEVO 7. How do I go about this? My understanding is that once I have the quad flying all I have to do is flip the switch and it starts the mission. Is this correct? Thanks