Grab the nearest book and turn to page 46. Copy the fifth sentence (and some following) on that page. For some reason, the book nearest to me right now is Spanish for Dummies, so here's mine:
In our example, she marries señor Juan José García Díaz. She is still called Fernández, but after father's name she adds de and her husband's surname, which is García. Now, she is señora María del Carmen Fernández de García.
Note that Spanish speakers capitalize señor and señora when abbreviated the way people in the States capitalize Mr. and Mrs.
And there you have it.