Let's talk about Meghan's new title, shall we? #RoyalWedding 
Today, the Queen gave Harry a title upgrade. The "promotion" means that he isn't known as HRH Prince Henry/Harry of Wales anymore (though people will still call him that). He is now known simply as HRH The Duke of Sussex, because British royals always use their highest title.
Meghan is now a princess, because she just married a prince (note the lowercase "p" in both cases). Because British royal women take their husband's rank and title, she is now a princess (rank) who is called HRH The Duchess of Sussex (title).
Her new title does not include her first name. (Harry's doesn't either.) She cannot be called Meghan, Duchess of Sussex. That form is used for women who are either widowed or divorced -- like Diana was at the time of her death.
Her first name is Meghan. Her rank is princess. Her title is HRH The Duchess of Sussex -- not Princess Meghan, or Duchess Meghan, or Meghan, Duchess of Sussex -- just HRH The Duchess of Sussex.
...EXCEPT when she's in Scotland. There, by tradition, the British royals use their highest Scottish titles. Meghan is HRH The Countess of Dumbarton when she crosses the border. (Camilla is HRH The Duchess of Rothesay when in Scotland, and Kate is HRH The Countess of Strathearn.)
...OR if she's in Northern Ireland! There the royals use their highest regional titles, too. Meghan is HRH Lady Kilkeel in Belfast. (Kate is HRH Lady Carrickfergus in Northern Ireland.)
If you're writing about Meghan, you can follow my lead. I find it easiest to call her "the Duchess of Sussex" first, and then say "Meghan" after that. No need for any awkward "Princess Meghan" or "Duchess Meghan" usages. Easy peasy!
(And if you're a writer who is worried about SEO, just pop "Meghan Markle" or "Princess Meghan" or whatever in your keywords/tags and call it good!)
