I am currently working on a full setup video for the QX90 as you are not thee only one asking questions about this but other things are contstantly getting in the way of me being able to finish the video!
However for now here is how I setup the flight modes on mine
Depending on how you setup your switched on your radio the channel might be aux1 or aux2, but this screen shows real time updates as you move it so you will know if its working correctly or not.
Also after you save its a good idea to disconnect the quadcopter and reconnect to make sure the settings save correctly as from time to time I find that it does not save my flight modes.
Also for now you might want to read our getting started guide for the falcon 180, its a different quad, but it shares the same flysky i6 radio and I talk about getting it all setup there.