How do you address royalty?

Mainly it depends on the country and its customs because some countries may not require a bow or curtsy to or addresing their members as a "Royal Highness"

In most royal houses when you greet any member of the royal family you are to either bow or curtsy and address with their respected royal titles and styles

The monarch and consort are to be addressed as "Your Majesty"

The Princes' and Princesses are to be addressed with either "Your Royal Highness or Imperial Highness" and in some situations both

After addressing the necessary titles you may (if told) address the person as "Sir" or "Ma'am"