Rule: You picked the wrong preposition, have too many prepositions, don't have enough prepositions, or put a preposition in the wrong place.
Examples:
Wrong: He earned a perfect score in the quiz.
Better: He earned a perfect score on the quiz.
Wrong: She fell off of the chair because she was laughing so hard.
Better: She fell off the chair because she was laughing so hard.
Wrong: Abigail holds a grudge to her town.
Better: Abigail holds a grudge against her town.
Wrong: If you walk the narrow path, you can get where you're going.
Better: If you walk on the narrow path, you can get where you're going.
Wrong: How do I know where you want to get to?
Better: How do I know where you want to go?
Practice using the correct prepositions:
Alejandra beat Mario into the top at the mountain, even though Mario went by kart.
When Adriana got off of the trampoline, she asked how soon she could take another turn in it.
Marquis knew all his multiplication tables by the middle on first grade.
Wang Li woke up in the crack of dawn to prepare for the long trip in Lakeville.
Resources for further explanations of prepositions:
Grammar Book's Prepositions
Purdue OWL's Prepositions