Thanks for asking! As you mentioned, flipping over a minor arcana card gives you the corresponding prompt. After you read the prompt, you also flip over a major arcana card to use as inspiration for answering the prompt. You do this process three times per act.
There's no additional prompt associated with the major arcana - the purpose is to provide an additional creative tool that you can use for inspiration. For example, let's say in act 1 I flip over the minor arcana 4 of Pentacles. I read that prompt, which is about a tattoo on my character's body. I then draw a major arcana card - in this case, lets say I get The Magician. I look over The Magician card, or optionally pull up a tarot guide, for inspiration. I decide to answer the prompt by saying the tattoo is a drawing of playing cards (in this case, I took inspiration from the name of The Magician). Then I move on to the next minor arcana, read that prompt, and draw a new major arcana for inspiration.
Let me know if you have any other questions - there's also a few examples of play in the book that might help clarify.