Some sentences don't say much.
(Who rode? What did they ride? Where did they go?)
You can expand the original sentence by answering these questions.
Kathy and Carlos rode on a giant book into a land of adventure.
To expand a sentence, answer some of the 5 W's and H—who, what, where, when, why, and how.
The magical book landed gently on a green hillside near the lair of a dragon.
(The new sentence answers what, how, and where. It even hints at why—the dragon!)