An interior monologue is one of the characters basically talking to themselves. The way you relate it to the book is by connecting it to the events of the story.
You might take some time to find a particular episode that Paul or Kat are involved in and then imagine what they would be saying to themselves while the action is taking place. You can talk about how terrified they might be (Paul is certainly not shy about doing this in the story) during the shelling or some other battle scene.
You need to make sure to use first person, obviously the character you choose would use "I," and you want to try and find an instance where the dialogue would be interesting because of some particular event or change in the story.