Breathe in, let the book go up.
Breathe out, let the book come down.
Notice whether you’re arching your back (no need for that, really).
Do this for as long as you like. Notice what you notice.
Work toward a silent breath in, an easy breath out, and maximum ease everywhere else. If it's easier with the book, use the book.
Let this kind of breathing become second nature to you. It will take practice, but five minutes at a time, once or twice a week will teach the body quickly. Then, you can try it standing up.
Quick recap: Lie on the floor with a book on your belly. Breathe in, the book goes up. Breathe out, the book goes down. Let the earth hold you up. There’s nowhere to be but here.