So she either picked up a practical disguise talent or learned how to transform "magically" without us ever seeing her being taught or considered important enough to be taught it.
There's a number of things we never see in the TV show, but are in the books. And then there are a number of things we never see or read. If we were to sit and watch every second of backstory and teaching of fighting, magic or otherwise, we'd still probably be waiting for Ned to lose his head.
