Unfortunately, the official site linked above has nada on setting up a Realm on Xbox One. But I did it and it's easy. You do it from within the game, in the "Store". It will give you the 2-friend and 10-friend options, as well as 30-day or 180-day renewal periods (there is a slight discount on the 180-day).
So far it works great - I can switch between Xbox and iOS seamlessly.