Write "INSERT" if you need the contents of a letter or book included in the screenplay for the reader, and then write in your prose style for that section. You should only do this if the prose is essential to the screen story.
Drop down one space and type the prose necessary to the scene. Use the "Action" paragraph setting for prose writing unless you're writing song lyrics or a poem. For these, indent to a "Dialogue" setting. Try to keep it as short as possible. Write in prose form with proper sentence case, in whatever tense the prose is written in. Writing temporarily in its own form also applies to a poem, song lyrics or words from a book.
Write "BACK TO SCENE" after the prose to show that the focus is going away from the inserted prose material. If the prose material is spoken by a character, simply write the prose material under a character heading as dialogue.