Good luck. Ask if you run into an obstacle, but it should all be there in the other threads.
Be aware that when system automation is enabled you will not be able to show the volume slider in the app, just the volume buttons. This is because system automation has no way to tell the NDX what the current volume is and you might change it with the volume knob on the unit, so it cannot show a slider with a particular position, only more/less with the buttons (which instructs the motor in the 252 to turn the poti right or left)
Personally, I find the buttons and slider cumbersome, so I use the remote for volume.
There is, by the way, also this current thread discussing whether the downsides (or not) of the variable control really exist: