You just place your aircraft in the aircraft folder of X-Plane 11.
Start X-Plane 11 and when loaded select New Flight and then select the airplane you want to fly.
If it is a new file in X-Plane 11 then it will show a " ? " for the aircraft, just continue with loading and when X-Plane has finished loading, go to the menu and select Developer, then select Regenerate icons for the current aircraft.
The next time you go to select or change to another aircraft you will be able to see liveries for the aircraft.
Each time you add a new livery unless it has the X-Plane generate icons you will need to "Regenerate the icons" each time you add anew one.
Cap't Kev